Ebook plant pathology george n agrios

George Ν Agrios Drawings by Mareile Fenner A C A D E M I C P R E S S N e w Y o r k S a n F r a n c i s c o L o n d o n A Subsidiary of Harcourt Brace Jovanovich, Publishers P L A I N T X | P A T H O L[.]

P L A I N T X | P A T H O L O G Y George Ν A g r i o s Department University of of Plant Pathology Massachusetts Drawings by Mareile Fenne r A C A D E M I C PRESS New York San Francisco A Subsidiary of Harcourt Brace Jovanovich, Publishers London C O P Y R I G HT © 9 , B Y A C A D E M IC P R E S S, I N C A LL R I G H TS R E S E R V E D N O P A RT OF T H IS P U B L I C A T I ON M AY B E R E P R O D U C ED OR T R A N S M I T T ED IN A NY F O RM OR B Y A NY M E A N S, E L E C T R O N IC OR M E C H A N I C A L, I N C L U D I NG P H O T O C O P Y, R E C O R D I N G, OR A NY I N F O R M A T I ON S T O R A GE A ND R E T R I E V AL S Y S T E M, W I T H O UT P E R M I S S I ON IN W R I T I NG F R OM T H E P U B L I S H E R A C A D E M IC P R E S S, I N C I ll Fifth Avenue, New York, New York 10003 United Kingdom A C A D E M IC Edition P R E S S, published I N C 24/28 Oval Road London NW1 by ( L O N D O N) L I B R A RY OF C O N G R E SS C A T A L OG C A RD N U M B E R: P R I N T ED IN T H E U N I T ED S T A T ES O F A M E R I CA L T D 69-18349 T o my wife Annette B r a y n a rd Agrios Preface T H E ADVANCES m a de in p l a nt p a t h o l o gy a nd the shift in e m p h a s is a nd direction this s c i e n ce has b e e n u n d e r g o i ng in the last s e v e r al years are the m a in factors r e s p o n s i b le for this text I m p o r t a nt items of information e l u c i d a t i ng p r o c e s s es of d i s e a se i n d u c t i on a nd d e v e l o pment, or d i s c o v e r i es in the a r e as of d i s e a se r e s i s t a n ce a nd control, h a ve b e e n a p p e a r i ng in i n c r e a s i ng n u m b e r s in the various j o u r n a ls of plant pathology T h e e x c i t e m e nt a nd significance of t h e se d e v e l o pm e n t s, h o w e v e r, e s c a pe the u n i n i t i a t ed s t u d e nt b e c a u se of the relative i n a c c e s s i b i l i yt of the original p a p e rs a nd of the difficult terminology e m p l o y ed in s u ch p a p e r s It is the p u r p o se of this b o ok to b r i ng together m u ch of this n e w information a nd to relate it to prev i o u s ly k n o wn facts so that it can b e logically p r e s e n t ed a nd e a s i ly u n d e r s t o od in an introductory c o u r se in p l a nt pathology T h e first part of the b o ok d e a ls with g e n e r al c o n s i d e r a t i o ns of d i s e a s e, the d i s e a se c y c l e, p a r a s i t i m s a nd p a t h o g e n i c i t y, a nd the variability in p a t h o g e n s T h is is f o l l o w ed b y a p r e s e n t a t i on of the m e c h an i s ms by w h i ch p a t h o g e ns c a u se d i s e a se a nd the m e c h a n i s ms b y w h i ch plants resist d i s e a s e C o n s i d e r a b le s p a ce is d e v o t ed to a bioc h e m i c al d i s c u s s i on of the effects of p a t h o g e n - p r o d u c ed e n z y m e s, toxins, growth regulators, a nd p o l y s a c c h a r i d es on the structural organization a nd on the b a s ic p h y s i o l o g i c al p r o c e s s es of photosynthesis, translocation, a nd respiration, as w e ll as to a b i o c h e m i c al d i s c u s s i on of the d e f e n se m e c h a n i s ms of the plant F i n a l l y, d i s c u ssions are i n c l u d ed on the g e n e t i cs of host-parasit e interaction, effects of e n v i r o n m e nt on d i s e a se d e v e l o p m e n t, a nd control T h e s e c o nd part of the b o ok d e a ls with the infectious d i s e a s es c a u s ed b y fungi, bacteria, parasitic h i g h er plants, v i r u s e s, a nd n e m avii Vlii PREFACE todes a nd with the noninfectious d i s e a s es c a u s ed by e n v i r o n m e n t al factors T h e d i s e a s es c a u s ed by e a ch t y pe of p a t h o g en are d i s c u s s ed c o m p r e h e n s i v e ly as a g r o up a nd are s u b s e q u e n t ly d i s c u s s ed individually in detail D i a g r a ms of c y c l es for e a ch d i s e a se are i n c l u d ed to h e lp the s t u d e nt create v i s u al i m a g es for the better a nd longerlasting u n d e r s t a n d i ng of the d i s e a s e As a r e s u lt of the greater knowle d ge a nd the significance of the viral a nd n e m a t o de d i s e a s es of plants, relatively m o re s p a ce has b e e n d e v o t ed to t h e se important d i s e a s es than w as customary heretofore I w o u ld like to e x p r e ss my gratitude to Mrs M a r e i le F e n n e r for he r patient work in p r e p a r i ng all d r a w i n gs in the book I also w i sh to thank Mr D Curtis a nd Mr L J M u s a n te for a s s i s t a n ec with the d r a w i n gs a nd with s o me of the p h o t o g r a p h s; D r s R A R o h d e, D F B a t e m a n, W M Banfield, J M D u n l e a v y, C J G i l g u t, F O H o l m e s, F W H o l m e s, W C M u e l l e r, A E Rich, a nd  M Z u c k e r m an for r e v i e w i ng the various parts of the m a n u s c r i p t; the p h o to library of the U n i t ed States D e p a r t m e nt of Agriculture, the D e p a r t m e nt of Plant Pathology of C o r n e ll University, a nd m a ny i n d i v i d u a l s, w h o se n a m es are g i v en in the pertinent l e g e n d s, for l e n d i ng m e p h o t o g r a p hs a nd for permitting m e to p u b l i sh them I gratefully a c k n o w l e d ge Mrs C a r ol Martula, Mrs M a r y a nn B e r e s t k a, Mrs Mary Martin, a nd Mrs S h i r l ey R o s es for their p a t i e n ce a nd p e r s e v e r a n ce in the s e e m i n g ly h o p e l e s s, yet successful, task of t y p i ng the m a n u s c r i p t, a nd the staff of A c a d e m ic Press for their cooperation in the p r o d u c t i on of this text G E O R GE N A G R I OS May, 1969 CHAPTER Introduction T H E W E L F A R E of plants is of particular interest to those m o st directly c o n c e r n e d with the growth of plants a nd the manufacture a nd distribution of plant products It is of c o n c e rn not only to farmers a nd workers in industries that p r o c e ss agricultural products, b ut a l so to innum e r a b le workers in s u p p o r t i ng industries w h o se l i v e l i h o od d e p e n ds on m a k i ng e q u i p m e nt or products u s ed in p r o c e s s i ng plant products — for e x a m p l e, m a c h i n e ry for textile industries — or on distributing the raw or m a n u f a c t u r ed agricultural products In the final analysis, however, the welfare of plants s h o u ld b e of c o n c e rn to e v e ry o ne of us as c o n s u m e rs of plants a nd of the e n d l e ss s e r i es of products d e r i v ed from plants T h e growth a nd y i e ld of plants d e p e n d on the availabilit y of nutrients a nd water in the soil w h e re they grow a nd on the m a i n t e n a n ce within certain r a n g es of such e n v i r o n m e n t al factors as light, temperature, a nd p H Plant growth a nd y i e ld d e p e n d also on protecting the plants from parasites Anything that affects the health of plants m ay affect their growth a nd y i e ld a nd m ay s e r i o u s ly r e d u ce their u s e f u l n e ss to t h e m s e l v es a nd to m a n k i n d Plant p a t h o g e n s, unfavorable weather, I INTRODUCTION a nd insect p e s ts are the m o st c o m m on c a u s es of reduction or destruction of plant growth a nd production Plants suffer from d i s e a s es w h o se c a u s es are similar to those affecting animals a nd m a n Although there is no e v i d e n ce that plants fee l p a in a nd discomfort, the d e v e l o p m e nt of d i s e a se follows the s a me steps a nd is u s u a l ly as c o m p l ex in plants as it is in animals a nd man Plant pathology is the study of (1) the living entities a nd the environmental conditions that c a u se d i s e a se in plants; (2) the m e c h a n i s ms by which t h e se factors p r o d u ce d i s e a se in plants; (3) the interactions b e t w e en the d i s e a s e - c a u s i ng a g e n ts a nd the d i s e a s ed plant; a nd (4) the m e t h o ds of p r e v e n t i ng d i s e a s e, alleviating the d a m a ge it c a u s e s, or controlling a d i s e a se either before or after it d e v e l o ps in a plant Plant pathologists study the d i s e a s es c a u s ed by fungi, bacteria, parasitic higher plants, viruses, a nd n e m a t o d e s T h e y also study plant disorders c a u s ed by the e x c e s s, i m b a l a n c e, or lack of certain physical or c h e m i c al factors, such as m o i s t u r e, t e m p e r a t u r e, a nd nutrients Plant d a m a g es c a u s ed by insects, m a n, or other a n i m a ls are not ordinarily i n c l u d ed in the study of plant pathology Plant pathology m ay call u p on the b a s ic t e c h n i q u es a nd k n o w l e d ge of botany, mycology, bacteriology, virology, nematology, plant anatomy, plant physiology, g e n e t i c s, biochemistry, horticulture, soil scie n c e , forestry, chemistry, p h y s i c s, m e t e o r o l o g y, a nd m a ny other b r a n c h es of s c i e n c e Plant pathology profits from a d v a n c es in any o ne of t h e se s c i e n c e s, a nd m a ny a d v a n c es in other s c i e n c es h a ve b e e n m a de in the attempt to solve phytopathologica l p r o b l e m s A g o od k n o w l e d ge of at least the b a s ic facts of the related s c i e n c es is indisp e n s a b le for efficient performance by any plant pathologist Although plant pathology as a s c i e n ce attempts to i n c r e a se our k n o w l e d ge of the c a u s es a nd the d e v e l o p m e nt of plant d i s e a s e s, it is also a s c i e n ce with a m o re practical goal T h e p u r p o se is to d e v e l op controls for all plant d i s e a s e s T h e goal is to s a ve the p r o d u ce w h i ch today is d e s t r o y ed by plant d i s e a s es a nd to m a ke it a v a i l a bel to the growers w ho toil to p r o d u ce it a nd to the hungry a nd ill-clothed millions of our increasingly o v e r p o p u l a t ed world T h e C o n c e pt of D i s e a se in Plants A plant is healthy or normal w h en it can carry out its p h y s i o l o g i c al functions to the b e st of its g e n e t ic potential T h e se functions i n c l u de normal cell division, differentiation, a nd d e v e l o p m e n t; absorption of water a nd minerals from the soil a nd translocatio n of t h e se throughout The Concept of Diseases in Plants the plant; p h o t o s y n t h e s si a nd translocatio n of the photosynthetic products to areas of utilization or storage; m e t a b o l i sm of s y n t h e s i z ed c o m p o u n d s; r e p r o d u c t i o n; a nd storage of food s u p p l i es for overwintering or reproduction W h e n e v e r plants are d i s t u r b ed by p a t h o g e ns or by certain environmental conditions a nd o ne or m o re of t h e se functions are interfered with b e y o nd a certain deviation from the normal, then the plants b e c o me d i s e a s e d T h e primary c a u s es of d i s e a se are either p a t h o g e ns or factors in the physical environment, b ut the specific m e c h a n i s ms by w h i ch d i s e a s es are p r o d u c ed vary c o n s i d e r a b ly with the causal a g e nt a nd s o m e t i m es with the plant At first the reaction of the plant to the d i s e a s e - c a u s i ng a g e nt is at the site of affliction, is of a c h e m i c al nature, a nd is i n v i s i b l e Soon, h o w e v e r, the reaction b e c o m es m o re w i d es p r e ad a nd histological c h a n g es take p l a ce that manifest t h e m s e l v es m a c r o s c o p i c a lyl a nd constitute the s y m p t o ms of the d i s e a s e Affected cells a nd t i s s u es of d i s e a s ed plants are u s u a l ly w e a k e n ed or d e s t r o y ed by the d i s e a s e - c a u s i ng agents T h e ability of such cells a nd t i s s u es to perform their normal p h y s i o l o g i c al functions is r e d u c e d or c o m p l e t e ly e l i m i n a t e d; as a result, plant growth is r e d u c e d or the plant d i e s T h e kinds of cells a nd t i s s u es that b e c o me infected determ i ne the t y pe of p h y s i o l o g i c al function of the plant that will b e interfere d with first T h u s, infection of the root (e.g., root rots) interferes with absorption of water a nd nutrients from the soil; infection of the xylem v e s s e ls (vascular wilts, certain cankers) interferes with translocation of water a nd m i n e r a ls to the crown of the plant; infection of the foliage (leaf spots, blights) interferes with p h o t o s y n t h e s i s; infection of the cortex (cortical canker, viral infections of p h l o e m) interferes with the d o w n w a rd translocatio n of photosynthetic p r o d u c t s; flower infections (bacterial blights, viral a nd fungal infections of flowers) interfere with reproduction; a nd infections of fruit (fruit rots) interfere with reproduction and/or storage of r e s e r ve foods for the n e w plant ( F i g 1) In contrast to the a b o v e, there is another g r o up of d i s e a s es in which the affected cells, i n s t e ad of b e i ng w e a k e n ed or d e s t r o y e d, are stimulated to d i v i de m u ch faster (hyperplasia) or to e n l a r ge a great d e al m o re (hypertrophy) than normal cells S u ch h y p e r p l a s t ic or hypertrop h i ed cells result in the d e v e l o p m e nt of u s u a l ly nonfunctioning, abnormally large, or abnormally proliferating organs or in the production of a m o r p h o us overgrowths on normal-looking organs O v e r s t i m u l a t ed cells a nd t i s s u es not only divert m u ch of the a v a i l a bel food stuffs to t h e m s e l v es a nd a w ay from the normal t i s s u e s, b ut frequently, by their e x c e s s i ve growth, crush a d j a c e nt normal t i s s u es a nd interfere with the p h y s i o l o g i c al functions of the plant Proteins synthesized Vitamins and hormones formed Shoot blight Reproduction and storage of starch, proteins,and fats j / ^ Transpiration 4 L fe a b lt i g h Fruit spot / Fruit rot Leaf spot Lighl Carbon dioxide I l Canker r translocation v Λ of water a ln c s/ ) i!I /) minerals l Μ /Food \, η translocation Photosynthesis (Food manufacture) \ Vascular wilt Crown gall Sugars and nitrogen form amino acids /// Uptake of water and minerals ^ < jtoot rot y v Protein synthesized F i g S c h e m a t i c r e p r e s e n t a t i o n of the b a s i c functions in a p l a n t a n d of the interfer­ e n c e with t h e s e functions c a u s e d b y s o m e c o m m o n t y p e s of p l a n t d i s e a s e s Importance of Plant Diseases—Losses D i s e a se in plants, then, can b e defined as any d i s t u r b a n ce brought a b o ut by a living entity or an environmental factor which interferes with manufacture, translocation, or utilization of food, mineral nutrients, a nd water in such a w ay that the affected plant c h a n g es in app e a r a n ce and/or y i e l ds l e ss than a normal, healthy plant of the s a me variety P a t h o g e ns m ay c a u se d i s e a se in plants by (1) c o n s u m i ng the contents of the host cells u p on contact; (2) killing, or d i s t u r b i ng the m e t a b o l i sm of host cells through toxins, e n z y m e s, or growth-regulating s u b s t a n c es they secrete, (3) w e a k e n i ng the host by continually a b s o r b i ng food from the host cells for their o wn u s e, a nd (4) b l o c k i ng the transportatio n of food, mineral nutrients, a nd water through the c o n d u c t i ve t i s s u e s D i s e a s es c a u s ed by environmental factors result from e x t r e m es in the conditions s u p p o r t i ng life (temperature, light, etc.) a nd in amounts of c h e m i c a ls a b s o r b ed or r e q u i r ed by plants T h e e x p r e s s i on of d i s e a se is the s um of b i o c h e m i c al reactions taking p l a ce at the point of contact of the p a t h o g en with the host cells or at the points of reaction of the e n z y m e s, toxins, etc., s e c r e t ed by the p a t h o g e n, with the host cells A w h o le plant, h o w e v e r, m ay b e c o me d i s e a s ed w h en only a small a r ea involving the m a in c o n d u c t i ve tiss u es of the plant is i n v a d ed by certain p a t h o g e ns a nd translocatio n failure results Although m an r e c o g n i z es plant d i s e a s es by the s y m ptoms or the unhealthy condition of affected plants, it s h o u ld b e rem e m b e r e d that d i s e a s es are not s y m p t o ms or conditions, nor are they the instigators of the s y m p t o ms or conditions, b ut rather they are the interactions b e t w e en host a nd p a t h o g en or b e t w e en host a nd a d v e r se environmental factors that result in s u ch s y m p t o ms or abnormal conditions T h u s, the p a t h o g en itself is not the d i s e a se —it is the c a u se of the d i s e a s e D i s e a se is the s um of the normal c h e m i c al reactions that are i n h i b i t ed a nd of the a b n o r m al c h e m i c al reactions i n d u c ed i n s i de the cells a nd in the t i s s u es of the plant as a result of the irritation b r o u g ht a b o ut b y the causal agent Importance of Plant Diseases —Losses Plant d i s e a s es are important to m an b e c a u se they c a u se d a m a ge to plants a nd plant products F or millions of p e o p le all over the world w ho still d e p e n d on their o wn plant p r o d u ce for their e x i s t e n c e, plant d i s e a s es can m a ke the difference b e t w e en a h a p py life a nd a life h a u n t ed by h u n g e r or can e v e n result in d e a th from starvation T h e d e a th from starvation of a quarter million Irish in 1845 a nd m u ch of the h u n g e r of the u n d e r f ed millions living in the u n d e r d e v e l o p e d, INTRODUCTION rural countries today are m o r b id e x a m p l es of the c o n s e q u e n c es of plant d i s e a s e s F or countries w h e re food is plentiful, plant d i s e a s es are important b e c a u se they c a u se e c o n o m ic l o s s es to growers, they result in i n c r e a s ed prices of products to c o n s u m e r s, a nd they destroy the b e a u ty of the e n v i r o n m e nt by d a m a g i ng plants a r o u nd h o m e s, a l o ng streets, in parks, a nd in forests Plant d i s e a s es m ay limit the kinds of plants that can grow in an a r ea or in an entire country by destroying all plants of certain s p e c i es that are extremely s u s c e p t i b le to a particular d i s e a s e, as e x e m p l i f i ed b y the A m e r i c an chestnut, w h i ch was annihilated in North A m e r i ca as a timber tree by the chestnut blight d i s e a s e Plant d i s e a s es m ay also d e t e r m i ne the kinds of agricultural industries a nd the level of e m p l o ym e n t in an a r ea by affecting the a m o u nt a nd kind of p r o d u ce a v a i l a bel for c a n n i ng or p r o c e s s i ng by the industries in the area On the other hand, plant d i s e a s es are r e s p o n s i b le also for the creation of n e w industries w h i ch d e v e l op c h e m i c a l s, machinery, a nd m e t h o ds to control plant d i s e a s e s; the annual e x p e n d i t u r es to this e n d a m o u nt to billions of dollars in the U n i t ed States alone T h e kinds a nd amounts of l o s s es c a u s ed by plant d i s e a s es vary with the plant or plant product, the p a t h o g e n, locality, environment, control m e a s u r es practiced, etc., or combinations of t h e se factors, a nd m ay range from slight loss to 0 % loss Plants or plant products m ay b e r e d u c e d in quantity by d i s e a se in the field, as i n d e ed is the c a se with most plant d i s e a s e s, or by d i s e a se d u r i ng storage, as is the c a se of the rots of stored fruits, v e g e t a b l e s, grains, a nd fibers F r e q u e n t l y, s e v e re l o s s es are c a u s ed by reduction in the quality of plant products F or instance, spots, s c a b s, b l e m i s h e s, a nd b l o t c h e s, on fruit, v e g e t a b l e s, or ornamental plants m ay h a ve little effect on the quantity p r o d u c e d, b ut the inferior quality of the p r o d u ct m ay r e d u ce the market v a l ue so m u ch that production is unprofitable or a total loss S o me d i s e a s e s, e.g., ergot of rye, m a ke plant products unfit for h u m an or animal cons u m p t i on by m a k i ng them p o i s o n o u s F i n a n c i al l o s s es resulting from plant d i s e a s es m ay b e incurred indirectly by the farmer's h a v i ng to plant varieties or s p e c i es of plants that are resistant to d i s e a se b ut are l e ss p r o d u c t i v e, or m o re costly, or c o m m e r c i a l ly less profitable; by his h a v i ng to spray or o t h e r w i se control a d i s e a s e, thus incurring e x p e n s es for c h e m i c a l s, machinery, stora ge s p a c e, a nd labor; by his h a v i ng to p r o v i de refrigerated w a r e h o u s es a nd transportatio n v e h i c l e s, thereby i n c r e a s i ng e x p e n s e s; b y limiting the time d u r i ng w h i ch products can b e k e pt fresh a nd healthy, thus forcing growers to sell d u r i ng a short p e r i od w h en products are abun- Classification of Plant Diseases dant a nd prices are low; by n e c e s s i t a t i ng the sorting of healthy from d i s e a s ed products, a nd thus i n c r e a s i ng costs of h a n d l i ng plant products S o me plant d i s e a s es can b e controlled a l m o st entirely by o ne or another m e t h o d, thus resulting in financial l o s s es only to the a m o u nt of the cost of the control S o m e t i m e s, h o w e v e r, this cost m ay b e almost as high as, or e v e n higher than, the return e x p e c t ed from the crop, as in the c a se of s o me d i s e a s es of small grains F or other d i s e a s es no effective control m e a s u r es are k n o wn as yet, a nd only a c o m b i n a t i on of cultural practices a nd s o m e w h at resistant varieties m a ke it p o s s i b le to raise a crop F or m o st plant d i s e a s e s, h o w e v e r, practical controls are a v a i l a bel although s o me l o s s es m ay b e incurred in spite of the control m e a s u r es taken In t h e se c a s e s, though, the benefits from the control a p p l i ed are g e n e r a l ly m u ch greater than the c o m b i n ed direct l o s s es from the d i s e a se a nd the indirect l o s s es d ue to e x p e n s es for control Classificatio n of Plant Diseases T h e r e are tens of t h o u s a n ds of d i s e a s es that affect cultivated plants On the a v e r a g e, e a ch kind of crop plant can b e affected by o ne hun d r ed or m o re plant d i s e a s e s E a ch kind of p a t h o g en m ay affect anyw h e re from o ne variety to s e v e r al d o z en or e v e n h u n d r e ds of s p e c i es of plants T o facilitate the study of plant d i s e a s e s, they m u st b e g r o u p ed in s o me orderly fashion T h is is n e c e s s a ry also for the identification a nd s u b s e q u e nt control of any g i v en plant d i s e a s e Any o ne of several criteria m ay b e u s ed as a b a s is for classificatio n of plant dise a s e s Plant d i s e a s es are s o m e t i m es classified a c c o r d i ng to s y m p t o ms they c a u se (root rots, cankers, wilts, leaf spots, s c a b s, blights, anthracn o s e s, rusts, smuts), a c c o r d i ng to the p l a nt organ they affect (root dise a s e s, s t em d i s e a s e s, foliage d i s e a s e s, fruit d i s e a s e s ), or a c c o r d i ng to the t y p es of plants affected (field crops d i s e a s e s, v e g e t a b le d i s e a s e s, fruit tree d i s e a s e s, forest d i s e a s e s, turf d i s e a s e s, d i s e a s es of ornamental plants) H o w e v e r, the m o st c o m m o n ly u s ed criterion is the type of p a t h o g en that c a u s es the d i s e a se (Fig 2) On this b a s is plant d i s e a s es are classified as follows: I I n f e c t i o us P l a nt D i s e a s es D i s e a s es c a u s ed b y fungi D i s e a s es c a u s ed b y b a c t e r ia D i s e a s es c a u s ed b y p a r a s i t ci h i g h er p l a n ts D i s e a s es c a u s ed b y v i r u s es D i s e a s es c a u s ed by n e m a t o d es F i g S c h e m a t i c d i a g r a m of the s h a p e s a n d s i z e s of c e r t a i n p l a n t p a t h o g e n s in relation to a p l a n t c e l l History of Plant Pathology I I N o n i n f e c t i o us or P h y s i o l o g i c al D i s o r d e rs N u t r i e nt d e f i c i e n c i es M i n e r al toxicities L a ck or e x c e ss of soil m o i s t u re T oo l ow or too h i gh t e m p e r a t u re L a ck or e x c e ss of l i g ht L a ck of o x y g en Air p o l l u t i on Soil a c i d i ty or alkalinity ( p H) History of Plant Pathology M an b e c a me painfully a w a re of plant d i s e a s es in the early times of antiquity T h is is e v i d e n c ed by the inclusion in the O ld T e s t a m e nt of b l a s t i ng a nd m i l d e w, a l o ng with h u m an d i s e a s es a nd war, a m o ng the great s c o u r g es of m a n k i n d T h e G r e ek p h i l o s o p h er T h e o p h r a s t us ( - 86 B.C.) w as the first actually to study a nd write a b o ut d i s e a s es of trees, c e r e a l s, a nd l e g u m e s, although his a p p r o a ch was observational a nd s p e c u l a t i ve rather than experimental H e noticed that differen t kinds of plants are m o re l i a b le to certain d i s e a s es than other plants, that e v e n varieties m ay differ in their susceptibility to a particular d i s e a s e, a nd that the soil a nd w e a t h er m ay influence the severity of a d i s e a s e H e b e l i e v ed that the d i s e a s es w e re p r o d u c ed spontan e o u s ly by the plant, b ut h e also s u g g e s t ed that, rarely, d i s e a s es m ay b e c a u s ed by creatures that not c o me from the plant itself, b ut from without E v en such creatures, h o w e v e r, w e re thought to h a ve arisen s p o n t a n e o u sy l in or on d e c o m p o s i ng matter D u r i ng the following 0 years, little w as a d d ed to the k n o w l e d ge of plant pathology, although reference s to the ravages of plant d i s e a s es a p p e a r ed in the writings of several contemporary historians T h e d i s c o v e ry of the c o m p o u nd m i c r o s c o pe a r o u nd the m i d d le of the 17th century o p e n ed a n e w era in the life s c i e n c e s T h e anatomy of plants w as s t u d i ed a nd d e s c r i b e d, a nd in 167 L e e u w e n h o e k, working with a m i c r o s c o pe h e h ad built, d i s c o v e r ed bacteria a nd m a ny other m i c r o o r g a n i s m s In 1729, M i c h e li u s ed the m i c r o s c o pe to study m a ny fungi a nd identified their " s e e d s" (spores) In a series of experim e n ts h e s h o w ed that w h en " s e e d s" of e a ch kind of fungus s t u d i ed w e re scattered on freshly cut surfaces of m e l o n s, q u i n c e, a nd p e a r s, they consistently p r o d u c ed crops of their o wn kind H e also p o i n t ed out that a few aberrant fungal growths on s e e d ed or u n i n o c u l a t ed control surfaces w e re the result of airborne s p o r es that h a p p e n ed to fall there 10 INTRODUCTION In 1755, T i l l et p r e s e n t ed the results of well-replicated a nd controlled plot e x p e r i m e n ts in w h i ch h e a d d ed the black d u st from b u n t ed w h e at to s e ed from healthy w h e at a nd o b s e r v ed that b u nt w as m u ch m o re p r e v a l e nt in plants p r o d u c ed from s u ch s e ed than from n o n d u s t ed s e e d H e thus s h o w ed that bunt, or stinking smut, of w h e at is a contagious plant d i s e a s e H e also s h o w ed that its o c c u r r e n ce can b e r e d u c e d by s e ed treatments Tillet, h o w e v e r, b e l i e v ed that it w as a p o i s o n o us s u b s t a n ce c o n t a i n ed in the dust, rather than living microorg a n i s m s, that c a u s ed the d i s e a s e P e r s o on (1801) a nd F r i es (1821) p u b l i s h ed p a p e rs on the taxonomy of fungi, b ut both b e l i e v ed that the rust a nd the s m ut fungi w e re products of the d i s e a s ed plants rather than distinct m i c r o o r g a n i s m s In 1807, Prevost p r o v ed c o n c l u s i v e ly that b u n t is c a u s ed by a fung u s; h e s t u d i ed the s p o r e s, their production a nd germination H e c o u ld control the d i s e a se by d i p p i ng the s e ed in a c o p p er sulfate solution, a nd h e p o i n t ed out the importance of the e n v i r o n m e nt in induction a nd d e v e l o p m e nt of the d i s e a s e Prevost's findings, h o w e v e r, w e re a h e ad of his time a nd w e re r e j e c t ed by a l m o st all his c o n t e m p oraries, w ho b e l i e v ed in s p o n t a n e o us generation T h e d e v a s t a t i ng e p i d e m i cs of late blight of potato in Northern E urope, particularly Ireland, in the 1840's tragically d r a m a t i z ed the importance of plant d i s e a s es a nd greatly s t i m u l a t ed interest in their c a u s e s T h e destruction of the potato crop in I r e l a nd in 1845 a nd 1846 c a u s ed w i d e s p r e ad famine which r e s u l t ed in the d e a th of h u n d r e ds of t h o u s a n ds of p e o p le a nd the immigration of m o re than o ne a nd a half million Irish to the U n i t ed States S e v e r al investigators d e s c r i b ed various a s p e c ts of the d i s e a se a nd of the p a t h o g e n, b ut it w as S p e e rs c h n e i d er (1857) a nd D e B a ry (1861, 1863) w ho finally p r o v ed experimentally that the fungus Phytophthora infestans is the c a u se of the d i s e a s e D e B a ry (1853), working at first with s m ut a nd rust fungi, e s t a b l i s h ed c o n c l u s i v e ly that fungi are c a u s e s, not results, of plant d i s e a s e H e d e s c r i b ed the m i c r o s c o p i c al structure a nd d e v e l o p m e nt of m a ny s m ut a nd rust fungi a nd the relationships of t h e se fungi to the t i s s u es of the d i s e a s ed plants D e B a ry also m a de great contributions with his studies of the P e r o n o s p o r a c e ae a nd the d i s e a s es they incite (downy m i l d e w s ), e s p e c i a l ly the late blight of potato, his d i s c o v e ry of the occurrenc e of two alternate hosts in the rusts, a nd his s t u d i es of the p h y s i o l o gy of the Sclerotinia rot d i s e a s es of carrots a nd other vegetab l e s In the Sclerotinia d i s e a s e s, D e B a ry n o t ed that host cells w e re killed in a d v a n ce of the i n v a d i ng h y p h ae of the fungus a nd that j u i ce from rotted t i s s ue c o u ld break d o wn healthy host t i s s u e B o i l ed j u i ce History of Plant Pathology from rotted t i s s ue h ad no effect on healthy t i s s u e D e B a ry c o n c l u d ed that the p a t h o g en p r o d u c es e n z y m es that d e g r a de a nd kill plant cells from which the fungus can then obtain its nutrients B e r k e l ey (1845, 1857) a nd K u hn (1858) p u b l i s h ed n u m e r o us p a p e rs on d i s e a s es of c e r e a l s, v e g e t a b l e s, a nd other plants T h e latter p u bl i s h ed the first textbook of plant pathology, in w h i ch climatic a nd soil conditions, insects, parasitic higher plants, a nd m i c r o o r g a n i s ms w e re listed as c a u s es of plant d i s e a s e s B r e f e ld (1875, 8 , ) contributed greatly to plant pathology by introducing a nd d e v e l o p i ng m o d e rn t e c h n i q u es for g r o w i ng microorg a n i s ms in p u re culture In this h e w as a s s i s t ed a great d e al by the m e t h o ds a nd refinements d e v e l o p ed b y Koch, Petri, a nd others Brefeld also s t u d i ed a nd illustrated the c o m p l e te life c y c l es of the s m ut fungi a nd d i s e a s es of cereal crops In 1878, a n e w d i s e a s e, the d o w ny m i l d ew of g r a p e, a p p e a r ed in E u r o p e T h e d i s e a se h ad b e e n i n t r o d u c ed from the U n i t ed States, s p r e ad rapidly, a nd t h r e a t e n ed to ruin the v i n e y a r ds of E u r o p e In 1882, Millardet n o t i c ed that v i n es w h i ch h ad b e e n s p r a y ed with the bluish-white mixture of c o p p er sulfate a nd l i me to deter pilferers ret a i n ed their l e a v es through the s e a s o n, w h e r e as the l e a v es of untreated v i n es h ad b e e n k i l l ed by the d i s e a se a nd h ad fallen to the ground After n u m e r o us s p r a y i ng e x p e r i m e n ts with various combinations of c o p p e r, c a l c i u m, a nd iron salts, M i l l a r d et finally c o n c l u d ed in 1885 that a mixture of c o p p er sulfate a nd h y d r a t ed l i me c o u ld effectively control the d o w ny m i l d ew of g r a p e T h is mixture b e c a me k n o wn as " B o r d e a ux M i x t u r e /' a nd its s u c c e ss in controlling d o w ny m i l d e ws a nd m a ny other foliage d i s e a s es w as spectacular E v en today B o r d e a ux mixture is the m o st w i d e ly u s ed fungicide all over the world T h e d i s c o v e ry of B o r d e a ux mixture g a ve great e n c o u r a g e m e nt a nd stimulus to the study of the nature a nd control of plant d i s e a s e s In the early 1900's, studies of the g e n e t i cs of d i s e a se r e s i s t a n ce in the cereal rusts b y Biffen (1905, 1912), a nd in the Fusarium wilts of cotton, w a t e r m e l o n, a nd c o w p ea by Orton (1900, 1909), l ed to the selection a nd b r e e d i ng of resistant varieties in t h e se a nd in other crops In the m e a n t i m e, Pasteur a nd K o ch h ad p r o v ed in 1876 that the animal d i s e a se anthrax is incited b y a b a c t e r i u m In 1878, Burrill s h o w ed that fire b l i g ht of p e ar a nd a p p le is also c a u s ed by a bacterium S o on after that several other plant d i s e a s es w e re s h o wn to b e c a u s ed by bacteria; t h e se i n c l u d ed the y e l l ow d i s e a se of hyacinth (Wakker, 1883), the olive knot d i s e a se (Savastano , 1887), a nd the bacterial wilt of cucurbits ( E F Smith, 1895) Smith's n u m e r o us a nd e x c e l l e nt con- 11 12 INTRODUCTION tributions in s u b s e q u e nt years to the study of bacterial d i s e a s es of plants e s t a b l i s h ed b e y o nd any d o u bt the importance of bacteria as p h y t o p a t h o g e n s T h e first plant parasitic n e m a t o d es w e re r e p o r t ed by N e e d h am in 1743 within w h e at galls (kernels) In 1855, B e r k e l ey o b s e r v ed rootknot n e m a t o d es in root galls of c u c u m b e r K u hn in 1857 d e s c r i b ed the b u lb a nd s t em n e m a t o de from m a l f o r m ed floral h e a ds of fuller's tassel, a nd in 1859 Schacht reported the sugar b e e t cyst n e m a t o d es from sugar b e e t roots A series of studies on plant parasitic n e m a t o d es w e re p u b l i s h ed by C o bb from 1913 to 1932 a nd contributed greatly to ne m a t o de taxonomy, morphology, a nd m e t h o d o l o g y In 1886, M a y er was working with a serious d i s e a se of tobacco which h e n a m ed " t o b a c co m o s a i c " H e c o u ld r e p r o d u ce the d i s e a se by inj e c t i ng j u i ce from infected tobacco plants into healthy plants T h e j u i ce of d i s e a s ed plants r e m a i n ed infective e v en after continual heating at ° C, although heating at - °C r e d u c e d its infectivity s o m ewhat a nd the j u i ce lost its infectivity after several hours of heating at ° C M a y er also noted that the j u i ce lost its infectivity after clarification a nd precipitation with a w e ak alcohol solution S i n ce no fungi w e re p r e s e nt on the d i s e a s ed plant or the filtered j u i c e, h e c o n c l u d ed that tobacco m o s a ic was p r o b a b ly c a u s ed by a b a c t e r i u m In 1891 , E F Smith s h o w ed that the p e a ch y e l l o ws d i s e a se was contagious, h ad a long incubation p e r i o d, a nd w as b u d transmitted, b ut h e c o u ld not d e t e r m i ne its c a u se although h e s u g g e s t ed that it was similar to that of tobacco mosaic In 1892, I v a n o w s ki s h o w ed that the causal a g e nt of tobacco m o s a ic c o u ld e v en go through a C h a m b e r l a nd filter that retains bacteria T h is led him to b e l i e ve that the d i s e a se w as c a u s ed by a toxin s e c r e t ed by bacteria or by small bacteria that p a s s ed through the p o r es of the filter Beijerinck (1898) was the first to c o n c l u de that tobacco m o s a ic w as c a u s ed not by a m i c r o o r g a n i sm b ut by a contagium vivum fluidum, which h e also c a l l ed a virus Beijerinck also found that the virus w o u ld infect a nd i n v a de y o u ng t i s s ue m o re rapidly than mature t i s s u e, that it m o v ed in the p h l o em a nd xylem, that it r e p r o d u c ed itself in the living plant, a nd that it c o u ld survive in d r i ed l e a v es a nd in the soil It was not until 1935, however, that the first major contribution was m a de r e g a r d i ng the nature of viruses T h at year Stanley o b t a i n ed a crystalline protein by treating j u i ce from infected tobacco plants with a m m o n i um sulfate H e c o u ld r e p r o d u ce the d i s e a se by inoculating healthy tobacco plants with that protein a nd c o n c l u d ed that the virus c o u ld b e c o n s i d e r ed as an autocatalyti c protein w h i ch c o u ld multiply Identification of Plant Diseases within living cells In 1936, B a w d en a nd his c o l l e a g u es d e m o n s t r a t ed that the crystalline preparations of the virus actually c o n s i s t ed of protein a nd n u c l e ic acid T h e first virus particles w e re v i e w ed with the electron m i c r o s c o pe b y K a u s c he a nd his c o l l e a g u es in 1939 In 1956, G i e r er a nd S c h r a mm s h o w ed that the protein c o u ld b e r e m o v ed from the virus a nd that the n u c l e ic a c id a l o ne c o u ld infect a plant a nd c o u ld r e p r o d u ce the c o m p l e te virus D u r i ng the 20th century, plant p a t h o l o gy has m a t u r ed as a s c i e n c e T h o u s a n ds of d i s e a s es h a ve b e e n d e s c r i b e d, p a t h o g e ns h a ve b e e n identified, a nd control m e a s u r es h a ve b e e n d e v e l o p e d T h e s t u d i es of g e n e t i cs a nd of the p h y s i o l o gy of d i s e a s es h a ve b e e n e x p a n d ed greatly, a nd n e w c h e m i c al c o m p o u n ds are b e i ng d e v e l o p ed continually to c o m b at plant d i s e a s e s Still, this is p r o b a b ly j u st the b e g i n n i ng of plant pathology a nd of the h o pe that it h o l ds for the future T h e h u g e l o s s es in plants a nd plant products that occur annually are the s i n g le b e st r e m i n d er of h ow m u ch is yet to b e l e a r n ed a b o ut plant d i s e a s es a nd their control T h e r e are t h o u s a n ds of plant d i s e a s es that we k n ow little or nothing about; there are p r o b a b ly n e w types of p a t h o g e ns that c a u se plant d i s e a s es a nd are a w a i t i ng discovery; our k n o w l e d ge of the p h y s i o l o gy of p l a nt d i s e a s es is dreadfully i n c o m p l e t e; a nd there m u st surely b e better materials a nd m e t h o ds for controlling plant d i s e a s es that are waiting to b e p r o d u c ed a nd d e v e l o p e d Progress in any a nd all of t h e se areas is the goal of plant pathology A n d a hungry, o v e r p o p ulated w o r ld is anxiously awaiting the results Identification of Plant Diseases In identification of any plant d i s e a se the first step is to d e t e r m i ne w h e t h er the d i s e a se is c a u s ed b y an infectious a g e nt or an environmental factor Infectious agents u s u a l ly p r o d u ce characteristic s y m ptoms on s o me parts of the plant that reveal the p r e s e n ce a nd s o m etimes the kind of the infectious agent S o me infectious a g e n t s, h o w e v e r, e s p e c i a l ly v i r u s e s, m ay p r o d u ce general s y s t e m ic s y m p t o ms very similar to those c a u s ed b y certain environmental factors If no p a t h o g en can b e found on or in a d i s e a s ed plant a nd if the s y m p t o ms p r e s e nt are not typical virus s y m p t o ms or cannot b e conside r e d as b e i ng c a u s ed by ectoparasitic n e m a t o d e s, environmental factors m ay b e c o n s i d e r ed as the c a u se of the d i s e a s e T o identify the environmental factor o ne w o u ld look for characteristic s y m p t o m s If no s u ch s y m p t o ms are present, then the e n v i r o n m e nt w o u ld b e exami n ed for u n u s u al conditions (e.g., low t e m p e r a t u r e, flooding) which 13 14 INTRODUCTION m ay h a ve e x i s t ed or for u n s o u nd cultural practices In s o me c a s es s p ecial tests m ay b e r e q u i r ed to d e t e r m i ne the p H of the soil, the prese n c e of phytotoxic minerals in high concentrations, etc Whe n a p a t h o g en is found on a plant, the p a t h o g en is identified by referenc e to s p e c i al m a n u a l s; a nd if the p a t h o g en is known to c a u se such a d i s e a s e, then the identification m ay b e c o n s i d e r ed c o m p l e t e d If, h o w e v e r, the p a t h o g en found s e e ms to b e the c a u se of the d i s e a s e, b ut no p r e v i o us reports exist to s u p p o rt this, then the following steps are taken to verify the h y p o t h e s is that the isolated p a t h o g en is the c a u se of the d i s e a s e: T h e p a t h o g en m u st b e found a s s o c i a t ed with the d i s e a se in all the d i s e a s ed plants e x a m i n e d T h e p a t h o g en m u st b e isolated a nd grown in p u re culture on nutrient m e d i a, a nd its characteristics d e s c r i b ed (nonobligate parasites), or on a s u s c e p t i b le host plant (obligate parasites), a nd its a p p e a r a n ce a nd effects r e c o r d e d T h e p a t h o g en from p u re culture m u st b e i n o c u l a t ed on healthy plants of the s a me s p e c i es or variety on w h i ch the d i s e a se a p p e a r s, a nd it m u st p r o d u ce the s a me d i s e a se on the i n o c u l a t ed plants T h e p a t h o g en m u st b e isolated in p u re culture again a nd its characteristics m u st b e exactly like those o b s e r v ed in step Whe n all the a b o ve steps (usually known as Koch's postulates) h a ve b e e n followed a nd p r o v ed true, then the i s o l a t ed p a t h o g en is identified as the o r g a n i sm a c c o u n t a b le for the d i s e a s e T h e steps for identification of virus d i s e a s es d e v i a te from those listed a b o ve o w i ng to the distinct nature a nd properties of viruses Whe n a plant is s u s p e c t ed to b e infected with a virus, proof that a virus c a u s es the d i s e a se can b e o b t a i n ed by transmitting the virus into healthy plants of the s a me s p e c i es or variety a nd r e p r o d u c i ng the s y m p t o ms of the d i s e a se in the i n o c u l a t ed plant T h e identification of the virus, h o w e v e r, is m a de by inoculating s e l e c t ed differential host plants w h i ch s e r ve as virus indicators a nd recording the s y m p t o ms p r o d u c ed on the indicators T h e identity of the u n k n o wn virus m ay b e d e t e r m i n ed by c o m p a r i s on of its s y m p t o ms on the indicators with those p r o d u c ed on the s a me indicators by k n o wn viruses M o re recently it has b e e n p o s s i b le to identify viruses by serological tests a nd other m e t h o d s CHAPTER Parasitism audi Disease Development T H E PATHOGENS that attack plants b e l o ng to the s a me g r o u ps of org a n i s ms (fungi, bacteria, v i r u s e s, n e m a t o d e s) that i n c l u de the pathog e n s that c a u se d i s e a s es in h u m a ns a nd animals With the e x c e p t i on of s o me insect-transmitted plant viruses, however, which c a u se d i s e a s es in both their host plants a nd their insect vectors, n o ne of the p a t h o g en s p e c i es that attack plants can affect h u m a ns or animals Plants are m o r e o v er attacked by a n u m b e r of other plants Infectious d i s e a s es are those that result from infection of a plant by a p a t h o g e n T h e y are characterized by the ability of the p a t h o g en to grow a nd multiply rapidly on d i s e a s ed plants a nd also by its ability to s p r e ad from d i s e a s ed to healthy plants and, thereby, to c a u se n e w d i s e a s e s Parasitis m a nd Pathogenicity Whe n an o r g a n i sm lives on or in s o me other o r g a n i sm a nd obtains its food from the latter, then it is aparasite T h e relationship b e t w e en 17 ... from plants T h e growth a nd y i e ld of plants d e p e n d on the availabilit y of nutrients a nd water in the soil w h e re they grow a nd on the m a i n t e n a n ce within certain r a n g... e, a nd nutrients Plant d a m a g es c a u s ed by insects, m a n, or other a n i m a ls are not ordinarily i n c l u d ed in the study of plant pathology Plant pathology m ay call u p on the... Importance of Plant Diseases—Losses D i s e a se in plants, then, can b e defined as any d i s t u r b a n ce brought a b o ut by a living entity or an environmental factor which interferes with manufacture,