Higher Plants

C H A P T E R 11

Plant Diseases Caused toy

Parasitic

Higher Plants

Introduction

More than 2500 species of higher plants are known to live parasiti- cally on other plants. T h e se parasitic plants produce flowers and seeds similar to those produced by the plants they parasitize. T h e y b e l o ng to several widely separated botanical families and show varying degrees of parasitism. Their parasitism is generally regarded as the result of a degenerative process by which plant species, which once were free- living and independent, lost the ability to carry out one or more of their physiological functions and are now d e p e n d e nt for their exis- tence on the host plant.

Characteristics of Parasitic Higher Plants

T h e parasitic higher plants vary greatly in their d e p e n d e n ce on their host plants. S o m e, usually called hemiparasites (e.g., orchids), 378

Dodder 379

have chlorophyll and roots and can, therefore, manufacture their own food from carbon dioxide and water but d e p e n d on their hosts for cer- tain dissolved minerals and probably s o me organic substances. Others (e.g., mistletoes) have chlorophyll but no roots and d e p e n d on their hosts for water and all minerals although they can produce all the car- bohydrates in their green leaves and stems. S o me other parasitic higher plants, however, having neither chlorophyll nor true roots, d e p e n d entirely on their hosts for their existence (e.g., dodder). Such complete parasites are never green, their leaves, if any, are usually small and inconspicuous, and their functional root system is greatly modified and d e v e l o ps entirely inside the host tissues.

As far as is known to date, parasitic higher plants cause d i s e a se mainly by depriving their hosts of the nutrients and water which are diverted and utilized by the pathogen. Although secretion of certain hydrolytic enzymes by s o me parasites has b e e n shown, and involve- men t of certain other chemical factors, e.g., growth regulators is sus- p e c t ed in d i s e a se development, the full mechanism of diseases c a u s ed by parasitic plants is still very unclear.

Important Groups of Parasitic Higher Plants

Relatively few of the known parasitic higher plants cause important d i s e a s es on agricultural crops or forest trees. T h e most common and serious parasites b e l o ng to the following botanical families and genera:

C o n v o l v u l a c e ae

G e n u s: Cuscuta, the d o d d e rs L o r a n t h a c e ae

G e n u s: Arceuthobium, the dwarf mistletoes of conifers

Phoradendron, the A m e r i c an true mistletoes of b r o a d l e a v ed trees Viscum, the E u r o p e an true mistletoes

O r o b a n c h a c e ae

G e n u s: Orobanche, the b r o o m r a p es of tobacco S c r o p h u l a r i a c e ae

G e n u s: Striga, the witch w e e ds of m a ny m o n o c o t y l e d o n o us plants

D o d d er

Occurrence and Importance

D o d d er is widely distributed in E u r o pe and in North America. In the United States it is most serious in the southern half of the country

380 11. DISEASES CAUSED BY HIGHER PLANTS

and in the North Central states, but crops like alfalfa and clover raised for s e ed may b e destroyed by dodder wherever they are grown. Other crops which suffer losses from dodder include lespedeza, onions, flax, sugar beets, several ornamentals, and potatoes.

D o d d er affects the growth and yield of infected plants and causes losses which range from slight to complete destruction of the crop in the infested areas. N a m e s, such as strangle w e e d, pull-down, hellbind, devil's hair, and hailweed, by which dodder is referred to in different areas, are descriptive of the ways in which dodder affects its host plants.

D o d d er may also serve as a bridge for transmission of viruses from virus-infected to virus-free plants as long as both plants are infected by dodder.

Symptoms

T h e first symptoms appear on the plants as fine, orange or yellow vine strands which grow and entwine around the stems and the other aboveground parts of the plants. D o d d er forms d e n se tangles of leaf- less strands on and through the crowns of the host plants (Fig. 77A).

T h e growing tips reach out and attack adjacent plants, until a grad- ually enlarging circle of infestation, up to 10 feet in diameter, is formed by a single dodder plant. Dodder-infested areas appear as patches in the field (Fig. 77B) which continue to enlarge during the growth season and, in perennial plants such as alfalfa, b e c o me larger every year. During late spring and in the s u m m er dodder produces m a s s ed clusters of white, pink, or yellowish flowers which soon form seed. T h e infected host plants b e c o me w e a k e n ed by the parasite, their vigor declines, and they produce poor yields. Many are pulled to the ground and may b e killed to the roots by the parasite. As the infec- tion spreads, several patches coalesce and large areas may b e formed which are easily seen by the yellowish color of the parasitic vine that covers them.

The Pathogen: Cuscuta sp.

T h e r e are many kinds of dodder plants which b e l o ng to different species of Cuscuta, but they all look very much alike and are difficult to distinguish from each other. T h e three most serious species of dod- der in the United States are: L a r g e s e ed dodder (Cuscuta indicora),

F i g. 77. (A) D o d d er on alfalfa. (B) Patches of d o d d er in a heavy infestation of an al- falfa field. (Photos by courtesy of U.S. D e p t. Agr.)

Dodder 381

382 11. DISEASES CAUSED BY HIGHER PLANTS

s m a l l s e ed dodder (C. planiflora), and field dodder (C. campestris).

T h e first two show d e c i d ed preference for l e g u m e s, but the third at- tacks many other broadleaf plants as well as legumes.

D o d d er is a slender, twining plant (Fig. 78). T h e stem is tough, curl- ing, threadlike and leafless, bearing only minute scales in place of leaves. T h e stem is usually yellowish or orange in color, sometimes tinged with red or purple; sometimes it is almost white. Tiny flowers m a s s ed in clusters occur on the stem from early J u ne until frost. Gray to reddish brown s e e ds are p r o d u c ed in a b u n d a n ce by the flowers and mature within a few weeks from bloom. T h e s e e ds fall to the ground and may germinate immediately or may remain dormant and germi- nate the following growth season.

Development of Disease

D o d d er s e ed overwinters in infested fields or m i x ed with the s e ed of crop plants. Sometimes dodder stems may also overwinter on de- bris on the ground. During the growing season the s e ed germinates and produces a slender yellowish shoot but no roots (Fig. 78). This leafless shoot rotates as though in search of a susceptible host. If no contact with a susceptible plant is m a d e, the stem falls to the ground, where it lies dormant for a few weeks and then dies.

If the dodder stem comes in contact with a susceptible host, howev- er, the stem immediately encircles the host plant, sends haustoria into it, and begins to climb the plant. T h e haustoria penetrate the stem or leaf and reach into the fibrovascular tissues. T h e haustoria secrete into the plant tissues enzymes that hydrolyze reserved food stuffs, such as starch, and thus make them available to the dodder plant. T h e se hy- drolyzed substances and water are absorbed by the haustoria and are transported to the dodder stem where they are utilized for further growth and reproduction.

Soon after contact with the host is established, the b a se of the dod- der shrivels and dries, so that it loses all connection with the ground and b e c o m es completely d e p e n d e nt on the host for nutrients and wa- ter. T h e dodder continues to grow and expand at the e x p e n se of the host and as it reaches the top or the periphery of the host plant the twisting tips of dodder reach out and attack adjacent plants. T h u s the infection spreads from plant to plant and patches of infected plants are formed. T h e growth of infected plants is s u p p r e s s ed and they may finally die.

In the meantime, the dodder plant has d e v e l o p ed flowers and pro- d u c ed seeds. T h e se fall to the ground where they either germinate

Fig. 78. Disease cycle of dodder (Cuscuta sp.) on alfalfa.

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adjacen t plant s Youn g dodde r seedling s rotatin g Germinatin g ^ dodde r see d > Dodde r see d | overwinterin g ^ in soi l

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384 11. DISEASES CAUSED BY HIGHER PLANTS

immediately or remain dormant until next season. T h e s e ed may b e spread to n e w nearby areas by animals, water, equipment, etc., and over long distances it is distributed mixed with the crop seed.

Control

D o d d er is best controlled by preventing its introduction into a field by the u se of dodder-free seed, by cleaning e q u i p m e nt thoroughly before moving it from dodder-infested fields to n e w areas, and by lim- iting the m o v e m e nt of domestic animals from infested to dodder-free fields.

If dodder is already present in the field, scattered patches may b e sprayed early in the season with contact herbicides such as diesel oil fortified with D N BP (4,6-dinitro-o-sec-butylphenol) or P CP (pentachlorophenol) or 2,4-D. Such treatment, or cutting or burning of patches, will kill both the dodder and the host plants, but will prevent dodder from spreading arid from producing seed. Whe n dodder infes- tations are already w i d e s p r e ad in a field, dodder can b e controlled by frequent tillage, flaming, and u se of soil herbicides such as C I PC

(isopropyl N-(3-chlorophenyl) carbamate), D C PA or Dacthal (dimethyl 2,3,5,6-tetrachloroterephthalate), dichlobenil or Casoron (2,6-dichlorobenzonitrile). T h e se chemicals kill the dodder plant upon its germination from the s e ed but before it b e c o m es attached to the host.

Selected References

D a w s o n, J. H., W. O. L e e , a nd F. L. T i m m o n s. 1965. Controlling d o d d er in alfalfa. U.S.

Dept. Agr. Farmer's Bull. 2 2 1 1 : 16 p p.

H a n s e n, A. A. 1 9 2 1 . D o d d e r. U.S. Dept. Agr. Farmers Bull. 1161.

M c N e e l y, G. Ç. , E. C. Hoffman, D. E. Bayer, a nd C. L. F o y. 1966. Control of d o d d er in alfalfa with D C P A, Calif. Agr. 2 0 (3): 14-16.

T h o d a y, Mary G. 1911 . On the histological relations b e t w e en C u s c u ta a nd its host. Ann.

Botany 2 5 : 6 5 5 - 6 8 2 .

Witchweed

Occurrence and Importance

Witchweed was known and recognized as a serious parasitic w e ed in Africa, Asia, and Australia before 1900. In 1956 the w e e d was dis-

Witchweed 385

covered for the first time in America, in the states of North and South Carolina. D ue to effective F e d e r al and State quarantines the spread of the parasite has b e e n largely limited to the area of the original infesta- tions.

A number of witchweed species parasitize important economic plants such as corn, sugarcane, rice, tobacco, and s o me small grains. At least 60 species in 28 gener a of the G r a m i n e ae are known to b e sus- ceptible to the one witchweed species found in the United States.

Witchweed causes its host plants to b e c o me stunted and chlorotic.

Heavily infected plants usually wilt and die. L o s s es vary with the d e g r ee of infestation in a field and may range from slight to 100%.

Symptoms

Affected plants d e v e l op symptoms r e s e m b l i ng those p r o d u c ed by acute drought. T h e plants remain stunted, wilt, and turn yellowish.

Death may follow these symptoms if the plants are heavily parasi- tized. Infected roots of host plants bear a large number of witchweed tentacles or haustoria which are attached to the root and fee d upon it.

Sometimes infected roots produce m a s s es of hairlike rootlets. One to several witchweed plants may b e growing above ground next to the infected plants, although roots of many more witchweed plants, which do not survive to reach the surface, may parasitize the roots of the same host (Fig. 79).

The Pathogen: Striga asiatica

Witchweed is a parasitic higher plant. It is a small, pretty plant with bright green stem and leaves. Both stem and leaves are slightly hairy.

T h e w e e d grows 15-30 c m high, although s o me specimens may reach a height of 45 cm. It produces multiple branches both near the ground and higher on the plant. T h e leaves are rather long and narrow and grow opposite each other in pairs, each pair b e i ng at 90-degree angle to the p r e c e d i ng one (Fig. 80).

T h e flowers are small and usually brick red or scarlet, although some may b e yellowish red, yellowish, or almost white, always having yellow centers. Flowers appear just above the leaf attachment to the stem and are p r o d u c ed throughout the season. After pollination s e ed pods or capsules develop, each containing more than a thousand tiny (about 200 μ long) brown s e e d s. A single plant may produce from 50,000 to 500,000 seeds.

T h e root of witchweed is watery white in color and round in cross

F i g. 79. Witchweed parasitizing roots of corn plant. (Photo by courtesy of U . S. D e p t.

Agr.)

386

Fig. 80. Disease cycle of witchweed (Striga asiatica) on corn. Witchweed capsule containing seeds Witchweed flower Part of corn plant Witchweed in bioom

Witchweed seed

Witchweed seeds in soil

Witchweed seeds germinating

Host plant

Root of ) host plant

^Witchweed rootlets produce haustoria on host plant root Haustorium /"< tracheids im- penetrate the\; vascular system b of host root V

Witchweed haustorium penetrates host cortex Rootlet

Young V witchweed^ plant ν Central core Df tracheids of haustorium ^Nucleus"

Host root vessels "Nucleus" of / haustorium \Witchweed growing Ji \on root 11 \of corn

388 11. DISEASES CAUSED BY HIGHER PLANTS

section. It has no root hairs, for it obtains all nutrients from the host plant through haustoria.

T h e life cycle of the parasite, from the time a s e ed germinates until the developing plant releases its first s e e d s, takes 90-120 days. T h e plant emerge s from the soil about 30 days after germination, it pro- duces flowers about 30 days after e m e r g e n c e , and the first s e ed pods burst about 30 days after flowering. Although after e m e r g e n c e the plant turns green and can probably manufacture some of its own food, it appears that it still continues to d e p e nd upon the host not only for all its water and minerals, but for organic substances as well.

Development of Disease

T h e parasite overwinters as s e e d s, most of which generally require a rest period of 15-18 months before germination, although s o me can germinate without any dormancy. Most s e e ds germinate only after stimulation by substances released from the roots of host plants. S e e ds within a few millimeters from host roots germinate and grow toward these roots, probably in response to stimulants contained in the exu- dates of the host roots. As soon as the w i t c h w e ed rootlet comes in con- tact with the host root, its tip swells into a conical or bulb-shaped haustorium which presses against the host root. T h e haustorium dis- solves host cells through enzymatic secretions and penetrates the host roots within 8-24 hours. T h e haustorium advances into the roots through dissolution of host cell walls. Finally its leading cells, usually tracheids, reach the vessels of the host roots (Fig. 80). T h e tracheids eventually dissolve the vessel walls or force their way into the vessel, into which they terminate bluntly or grow lengthwise in the xylem, from which they absorb water and nutrients. Although xylem vessels are present in the haustorium, no typical p h l o em cells develop, but cells in the " n u c l e u s" of the haustorium s e em to connect the phloem of host and parasite. It has b e e n shown that the chlorophyll of stems and leaves of witchweed plants is functional and therefore the w e ed can synthesize s o me of its own foodstuffs. It is also known, however, that manufactured foodstuffs move from the host plant into the para- site even when the latter is fully developed.

From the initial rootlet the w e ed produces more roots which move parallel to the roots of the host plant and s e nd more haustoria into them. Furthermore, several hundred separate witchweed plants may parasitize the roots of a single host plant at once, although relatively

Witchweed 389

few of these survive to reach the surface b e c a u se the host plant cannot support so many.

T h e d i s e a se spreads in the field in a circular pattern. T h e center of the infestation b e c o m es so devastated that few hosts, and conse- quently few witchweeds, can grow. Surrounding this area there is a b a nd of yellowish host plants heavily parasitized by witchweed, and farther out the witchweed population diminishes and the appearance of the host plants improves. T h e circle of infected plants, however, increases year after year as the witchweed s e e ds spread in increas- ingly larger areas. T h e s e e ds are spread by wind, by water, by contam- inated tools and equipment, or by contaminated soil carried on farm machinery.

Control

Witchweed can b e controlled by preventing its movement from in- fested areas into uninfested ones on transplants, agricultural products and machinery, or in any other way. Catch crops, consisting of host plants, may b e planted to force the germination of witchweed seed, and the witchweed plants then can b e destroyed by plowing under or by the u se of w e e d killers such as 2,4-D. T r ap crops, consisting mostly of nonhost l e g u m e s, may b e u s ed to stimulate germination of witch- w e ed s e e d s, which, however, cannot infect the trap plants and there- fore starve to death. Usually, a combination of the above methods is required to prevent witchweed plants from flowering and seeding, but with appropriate timing and perseverence an area can b e completely freed from the parasite.

Selected References

A n o n y m o u s. 1957. Witchweed. U.S. Dept. Agr., Agr. Res. Serv. Spec. Rept. 2 2 - 4 1 , 1 7 pp.

Garriss, H. R., a nd J. C. Wells. 1956. Parasitic h e r b a c e o us annual a s s o c i a t ed with corn d i s e a se in North Carolina. Plant Disease Reptr. 40: 8 3 7 - 8 3 9 .

N e l s o n, R. R. 1958. Preliminary studies on the host range of Striga asiatica. Plant Dis- ease Reptr. 4 2 : 3 7 6 - 3 8 2 .

Rogers, W. E., a nd R. R. N e l s o n. 1962. Penetration a nd nutrition of Striga asiatica. Phy- topathology 52 : 1064-1070.

S a u n d e r s, A. R. 1933. S t u d i es in p h a n e r o g a m ic parasitism with particular referenc e to Striga lutea. Union South Africa Dept. Agr. Set. Bull. 128.

390 11. DISEASES CAUSED BY HIGHER PLANTS

Dwarfmistletoes of Conifers Occurrence and Importance

Dwarfmistletoes occur in all parts of the world where conifer trees grow. In the United States they are more prevalent and most serious in the western half of the country, especially in the states along the Pacific coast, but they also cause appreciable losses in the northeast- ern and southeastern states.

T h e d a m a ge c a u s ed by dwarfmistletoes in coniferous forests is ex- tensive although not always spectacular. Tree s of any age may b e re- tarded, deformed, or killed. Height growth of trees may b e r e d u c ed by 5 0 - 8 0 %. T i m b er quality is r e d u c ed by numerous large knots and by abnormally grained, spongy wood. Seedlings and saplings, as well as trees of certain species, are frequently killed by dwarfmistletoe infec- tions.

Symptoms

Infected plants bear simple or branched shoots of dwarfmistletoe plants which occur in tufts or scattered along the twigs of the host (Fig. 81). If the shoots have d r o p p ed off, small basal " c u p s" appear on the bark. Infected twigs and branches d e v e l op swellings and cankers on the infected areas. Cross sections at the swellings of infected branches reveal yellowish, w e d g e - s h a p ed haustoria of the parasite which grow into the bark, cambium, and xylem of the branch. Several susceptible species frequently respond to infection by a proliferous growth of branches which appear as witches' brooms, while the unin- fected parts of the tree deteriorate markedly. L a r ge swellings or flat- tened cankers may also d e v e l op on the trunks of some infected trees.

T r e e stands with light or moderate infections are difficult to distin- guish from healthy stands except for the presence of cankers, swell- ings, and brooms. Heavily infected stands, however, contain de- formed, stunted, dying, and d e ad trees, or trees broken off at trunk cankers, and give the appearance of poor site conditions.

The Pathogen: Arceuthobium sp.

T h e r e are many different species of Arceuthobium that parasitize conifers. S o me of these produce shoots up to 10 cm long, while the others are usually no more than 1.5 cm. In general the various species are confined to a definite host or group of hosts, but transfers from one host species or genus to another are not uncommon.

Dwarfmistletoes of Conifers

F i g. 8 1 . (A) D w a rf mistletoe plants on a nine-foot p o n d e r o sa p i n e. A female plant in center, a m a le plant on the s i de limb. (B) D w a rf mistletoe shoots parasitizing branches of l o d g e p o le p i n e. N o te the s w e l l i ng a nd distortion of the affected parts of b r a n c h e s. A few small basal " c u p s" can also b e s e en on the p i ne branch. (Photos by courtesy of U . S. F o r e st Service.)

391

392 11. DISEASES CAUSED BY HIGHER PLANTS

T h e mistletoe stems vary in color from yellowish to brownish green or olive green in many shades. T h e shoots may b e simple or branched and they are jointed. T h e leaves are inconspicuous, scalelike, in oppo- site pairs, and of the same color as the stem. Mistletoe plants also pro- duce a complex, ramifying system of haustoria which consists of a lon- gitudinal system of strands, external to and more or less parallel to the host cambium, and a radial system of " s i n k e r s" produced by the former and oriented radially into the phloem and xylem.

T h e plants are either male or female, and produce flowers when they are 4-6 years old. After flowering the male shoots die; the female shoots die after the seeds are discharged. Fruits mature 5-16 months after pollination of the flowers. T h e fruit at maturity is a recurved, tur- gid, elliptical berry. On ripening, the fruit develops considerable in- ternal pressure and, when disturbed, expels the s e ed upward or ob- liquely at lateral distances up to 50 feet. T h e s e ed is covered with a mucilaginous sticky substance and adheres to whatever it comes in contact with. This is the main means of spread of the parasite.

Development of Disease

Whe n a mistletoe s e ed lands on and b e c o m es attached to the bark of a twig or a young branch of a susceptible host, it germinates and pro- duces a germ tube or radicle. This grows along the bark surface until it meets a b ud or a leaf b a s e, at which point the radicle b e c o m es broad and flattened on the side of the bark. A rootlike haustorium is then produced from the center of the flattened area of the radicle, which penetrates the bark directly and reaches the phloem and the cam- bium. From this haustorium develops the system of longitudinal strands and radial sinkers, all of which absorb from the host the nutri- ents n e e d e d for the development of the parasite (Fig. 82). T h e strands that reach the c a m b i um of the host b e c o me permanently e m b e d d ed in the wood as the latter is laid down each year, but they always retain their connections with the strands in the phloem. After the haustoriall system is well established and d e v e l o p ed in the host, it produces b u ds from which shoots d e v e l op the following year or several years later.

T h e shoots first appear near the original point of infection, but later more shoots e m e r ge in concentric zones of increasing diameter. T h e center of the infection usually deteriorates and b e c o m es easily at- tacked by various decay-producing fungi. If witches' brooms are pro- d u c ed on the affected area, the haustoria pervade all branches and produce mistletoe shoots along the proliferating host branches. T h e haustorial strands and sinkers originate as one-cell filaments with a

Fig. 82. Disease cycle of dwarf mistletoe (Arceuthobium sp.) on conifers.

Expelled seeds land on conifer twigs and branches The germinating seed . produces a haustorium ]which penetrates the bark dwarf mistletoe overwinters as plants and seeds on conifers ^ ^ Cross section of * infected twig. C=Cups; S= Sinkers; LS=Longitudinal strands; NS=New shoot / Advanced ψ infection

Cups from ) 1 iallen shoots /

Pine twig < infected with 4 dwarf mistletoe )i Female plant V with seeds \

Dwarf mistletoe r'r>>sseed Female plant in bloom Pine branch heavily infected^ with ^ dwarf mistletoe j

Haustorial strands 3nd sinkers

Male plant in bloom Dwarf mistletoe shoots

394 11 . DISEASES CAUSED BY HIGHER PLANTS

single apical cell, but divisions in the more distal cells turn the strands into rootlike structures with vascular tissues. Quite frequently direct xylem-to-xylem connections d e v e l op b e t w e en haustoria vessels a nd host tracheids through which absorption b y the parasite of nutrients and water takes place.

T h e parasite removes nutrients from the host and so starves and kills the portion of the branch lying b e y o nd the point of infection. It also saps the vitality of the branch and, when sufficiently abundant, of the whole tree. Furthermore, it upsets the balance of hormonal sub- stances of the host in the infected area and causes hypertrophy a nd hyperplasia of the cells with resulting swellings a nd deformities of various shapes on the branches. This hormonal imbalance also stimu- lates the normally dormant lateral b u ds to excessive formation of shoots, forming a d e n se growth of abnormal appearance. H e a vy dwarf- mistletoe infections weaken trees a nd predispose them to wood d e- caying and root pathogens, to beetles, and to windthrow and breakage.

Control

T h e only means of controlling dwarfmistletoes is by physical re- moval of the parasite. This is done either by pruning infected branches or b y cutting and removing entire infected trees. Uninfected stands can b e protected from dwarfmistletoe infections b y maintain- ing a protective zone free of the parasite b e t w e en the d i s e a s ed stand and the stand to b e protected.

Selected References

C o h e n, L. I. 1954. T h e anatomy of the e n d o p h y t ic s y s t em of the dwarf mistletoe, Arceu- thobium campylopodum. Am.]. Botany 4 1 : 8 4 0 - 8 4 7 .

Gill, L. S. 1935. Arceuthobium in the U n i t ed States. Conn. Acad. Arts Set. Trans. 3 2 : 111-245.

K i m m e y, J. W. 1957. Dwarfmistletoes of California a nd their control. Calif. Forestry Range Expt. Sta., Berkeley, Tech. Paper 19: 12 p p.

Kuijt, J. 1955. Dwarfmistletoes. Botan. Rev. 2 1 : 5 6 9 - 6 2 8 .

Kuijt, J. 1960. Morphological aspects of parasitism in the dwarf mistletoes (Arceutho- bium). Univ. Calif. Publ. Botany 3 0 : 3 3 7 - 4 3 6 .

Roth, L. F. 1954. Distribution, s p r e ad a nd intensity of dwarf mistletoe on P o n d e r o sa p i n e. Phytopathology 44: 504.