^Zoospores
\ Zoosporangiu
m
^Zoosporangium Zoosporangium
Sporangiophor e
onlea f
Zoosporangiophore on
infecte d tube r ^
\ (in spring) j
Γ Zoosporangiophore
~o n infecte d
^seedlin g / Myceliu m
fromtube r •infect s A$e_edlin g
l|xAntheridiumOogonium
Antheridiu m
Onnoniiim
Oospore /
Germination /
J>
Sporangiu m
effect of humidity and temperature on the different stages of the life cycle of the fungus. T h e fungus sporulates most abundantly at relative humidity of, or near 1 0 0 % and at temperatures b e t w e en 16° and 22°C.
Sporangia lose their viability in 3-6 hours at relative humidities below 8 0 %. Germination of sporangia takes place only when free water or d ew is present on the leaves and, at 10°-15°C, it may be completed within 0.5-2 hours. After germination a period of 2-2.5 hours at 15° to 25°C is required for penetration of the germ tubes into the host tissue.
After penetration, the mycelium develops most rapidly at 17°-21°C, which is also optimal for sporulation. Temperatures above 30°C chec k the growth of the fungus in the field but do not kill it, and the fungus can start to sporulate again when the temperature b e c o m es favorable, provided of course that the relative humidity is sufficiently high.
Control
Late blight of potatoes can be successfully controlled by a combina-tion of sanitary measures, resistant varieties, and well-timed chemical sprays. Only disease-free potatoes should be u s ed for seed. Potato d u m ps or cull piles should b e b u r n ed before planting time in the spring or sprayed with strong herbicides to kill all sprouts or green growth. All volunteer potato plants in the area, whether in the potato or other fields should b e destroyed, since any volunteer potato plant can be a source of late blight infection.
Only the most resistant potato varieties available should b e planted.
Such varieties include Boone, Catoosa, Cherokee, E s s e x, F u n d y, K e n n e b e c , Onaway, Plymouth, etc. T h e blight fungus has a number of races or strains differing from each other in the potato varieties that they can attack. Several potato varieties resist one or more races of the late blight fungus. S o me of them have resistance to vine infection, but not to tuber infection. N ew varieties, derived from crosses with So-lanum demissum, have withstood attack by all known races of the fun-gus for a while, but were finally attacked by other races not previously distinguished or perhaps not previously existent. Many varieties pos-sess so-called "field resistance," which is only a partial resistance of varying degrees but which is effective against all races of the blight fungus. However, it is not sufficient to rely on varietal resistance to control late blight since, in favorable weather, blight can severely in-fect these varieties unless they are sprayed with a good protective fungicide. E v en resistant varieties should b e regularly sprayed with fungicides to eliminate, as much as possible, the possibility of becom-ing suddenly attacked by races of the fungus to which they are not re-sistant or by entirely ne w races.
Late Blight of Potatoes 247
Chemical sprays with fungicides, if a p p l i ed properly, generally will k e e p late blight under control. Spraying should start when potato plants are 15-30 cm high or at least 10 days before the date late blight usually appears in the area. Sprays should b e a p p l i ed once every 4 -5 days when the weather is damp, misty, or rainy and when the nights are cool, and should continue until the foliage dies naturally or is killed artificially by 'Vine-killers." Proper timing and thorough cover-age of old and ne w folicover-age are essential if plants are to b e protected from the disease. Once late blight b e c o m es established it is extremely difficult to control unless the weather turns hot (35°C and above) and dry. Materials u s ed for late blight control include several dithiocarba-mates, such as Dithane M-45, m a n eb and nabam usually mixed with zinc sulfate to form a tank-mix zineb, and several copper materials, such as copper oxychloride, and Bordeaux mixture. Protective spray-ing of foliage usually effects a very considerable reduction in tuber infection. When , however, partially blighted leaves and stems are still surviving at harvest time, it is necessary to remove the aboveground parts of potato plants or destroy them by chemical sprays or mechani-cal means. Herbicides u s ed for this purpose include copper sulfate, sodium and potassium arsenites, sulfuric acid, and certain dinitro compounds. po-tato (Phytophthora infestans). Wisconsin Univ. Agr. Expt. Sta. Res. Bull. 27: 64 p p.
Reddick, D., and W. Mills. 1938. B u i l d i ng u p virulence in Phytophthora infestans. Am.
Potato J. 15: 29-34.
Romero, S., a nd Ì . E. Gallegly. 1963. O o g o n i um germination in Phytophthora infes
tans. Phytopathology 53: 8 9 9 - 9 0 3 .
Weihing, J. L., and R. B. O'Keefe. 1962. E p i d e m i o l o g i c al potentials of potato varieties in relation to late blight. Phytopathology 52: 1268-1273.
D o w ny M i l d ew of G r a pe Occurrence and Importance
D o w ny mildew of grapes is one of the many downy mildew dis-eases that affect a variety of vegetables, ornamentals, cereals, and other crop and w e ed hosts, particularly in cool, humid environments.
D o w ny mildew of grape occurs in most parts of the world where grapes are grown under humid conditions. It is most destructive in E u r o pe and in the eastern half of the United States, where it may cause severe epiphytotics year after year, but it is also known to have caused serious losses in some years in northern Africa, in South Africa, in parts of Asia, Australia, and South America. Dry areas like Califor-nia are free of the disease.
D o w ny mildew affects the leaves, fruit, and vines of grape plants and causes losses through killing of leaf tissues and defoliation, through production of low quality, unsightly, or entirely destroyed grapes, and through weakening, dwarfing, and killing of young shoots.
Whe n weather is favorable and no protection against the disease is provided, downy m i l d ew can easily destroy 5 0 - 7 5 % of the crop in one season.
Symptoms
T h e d i s e a se is usually first observed as small, translucent, pale yel-low spots with indefinite borders on the u p p er surface of the leaves, while on the under surface of the leaves, and directly under the spots, a downy growth of the sporophores of the fungus appears (Figs. 27 and 28A), which may b e more or less obvious d e p e n d i ng on the number of leaf hairs present on the underside of the leaves of the grape variety.
Later, the infected leaf areas are killed and turn brown, while the spo-rophores of the fungus on the under surface of the leaves b e c o me dark gray. T h e necrotic lesions are irregular in outline, and as they enlarge
Downy Mildew of Grape 249
F i g. 27. D o w ny m i l d ew on g r a pe leaves a nd fruit. (Photo by courtesy of the Depart-m e n t of Plant Pathology, Cornell University.)
they may coalesce to form large d e ad areas on the leaf, frequently re-sulting in defoliation.
During blossom or early fruiting stages, entire clusters or parts of them may b e attacked, are quickly covered with the downy growth,
F i g. 28. (A) D o w ny m i l d ew on the u n d e r s i de of a g r a pe leaf. (B) T i ps of g r a p e v i ne c o v e r ed with d o w ny m i l d e w. (Photos by courtesy of U.S. D e p t. Agr.)
and die. If infection takes place after the berries are half-grown, the fungus grows mostly internally, the berries b e c o me leathery and somewhat wrinkled and d e v e l op a reddish-marbling to brown colora-tion.
Infection of green young shoots, tendrils, leaf stems, and fruit stalks results in stunting, distortion, and thickening (hypertrophy) of the tis-sues. Entire shoots may b e covered with the downy growth of the fun-gus (Fig. 28B). Later the funfun-gus growth breaks down and disappears, and the infected tissues turn brown and die. In late or localized infec-tions the shoots may not b e killed, but they show various d e g r e es of distortion.
The Pathogen: Plasmopara viticola
T h e pathogen is a phycomycete. T h e mycelium is continuous, al-though older hyphae may have occasional cross walls. T h e diameter of the mycelium varies from 1 to 60 ì b e c a u se the hyphae take up, and conform to, the shape of the intercellular spaces of the infected tis-sues. T h e mycelium grows b e t w e en the cells but sends numerous, globose haustoria into the cells (Fig 29). In humid weather the myce-lium produces sporangiophores which e m e r g e on the under side of
Fig. 29. Disease cycle of downy mildew of grape caused by Plasmopara viticola. Oospore Oogonium
Antheridium
Oospore inside infected leaves
j Oospore on the ground l Germinating oospore
Tlnfected _ leaf /
InterceHular mycelium haustoria /Infected f grape cluster / lnfected\ twig / Sporangium
\ Germinating / sporangium Sporangiophore Mycelium in dormant twig
f Sporangium
Germinating sporangium^ Zonsnorp /
Encysted zoospore Zoospore Leaf^ infection ^ Berry infection Twig infection
the leaves and on the stems through the stomata or rarely by pushing directly through the epidermis. In the young fruit, sporangiophores emerge through lenticels. Usually 4-6 sporangiophores arise through a single stoma, but sometimes there may b e as many as 20. Sporangio-phores are 7-9 μ in diameter and 500 μ or more in length. E a ch pro-duces 4-6 branches at nearly right angles to the main stem of the spo-rangiophore, and each branch produces 2, sometimes 3, secondary branches in a similar manner. At the tips of the secondary branches, single, lemon-shaped sporangia (conidia) are produced. T h e sporan-gia are transported by water or are blown by the wind and in the pres-enc e of free moisture they germinate. Sporangia generally germinate by means of zoospores, which e m e r g e from the sporangia, swim about for a few minutes, encyst, and then produce a germ tube by which they can infect the plant. In rare instances sporangia may germinate by directly producing a germ tube.
T h e fungus also produces oospores, presumably through the fertili-zation of oogonia by antheridia, although antheridia have seldom b e e n observed. Oospores are commonly formed in the leaf tissues and sometimes in berries and in the cortex of the shoots. Oospores germi-nate by producing a germ tube which termigermi-nates in a sporangium. T h e sporangium then germinates by means of zoospores, as described above.
Development of Disease
T h e pathogen overwinters as oospores e m b e d d ed in the d e ad leaves, and occasionally, in d e ad berries and shoots (Fig. 29). On some grape species, and in certain areas, the fungus may also overwinter as mycelium in infected, but not killed, twigs. T h e d e ad leaves contain-ing the oospores disintegrate durcontain-ing the winter and liberate the oo-spores. During rainy periods in the spring the oospores germinate ei-ther on the ground or on parts of grape plants on which they are carried by wind or splashing rain drops. T h e p r o d u c ed sporangium or its zoospores are transported by wind or water to the wet leaves near the ground which they infect. Penetration takes place through stomata of the lower surface of the leaves. T h e mycelium then spreads into the intercellular spaces of the leaf, obtaining food through globose hausto-ria which it sends into the cells. T h e m y c e l i um continues to spread into the tissues and w h en it reaches the substomatal cavity it forms a cushion of mycelium from which the sporangiophores arise and e m e r ge through the stoma. T h e chlorophyll of infected parenchyma cells begins to disappear: the plastids and eventually the nucleus of
Downy Mildew of Grape 253
the cells disintegrate. As the infection advances, the cell contents b e c o me plasmolyzed, the cell walls collapse, and the entire cells turn brown. On these primary lesions great numbers of sporangia are pro-d u c epro-d which may b e carriepro-d, by winpro-d or rain, to nearby healthy plants. Whe n the sporangia land on a wet leaf, cluster, or shoot of the grape plant they germinate quickly and produce many zoospores. T h e zoospores then cause secondary infections through stomata or lenti-cels and thus rapidly spread the disease. T h e period from infection to ne w sporangia formation varies from 5 to 18 days, d e p e n d i ng on tem-perature, humidity and varietal susceptibility. Under average temper-ature conditions for grape-growing areas the number of d i s e a se cycles per season on susceptible varieties d e p e n ds only on the frequency of rainfall and the duration for which the green tissues remain wet each time.
In the stems the fungus invades the cortex, ray parenchyma, and pith. T h e distortion and hypertrophy of infected stems is c a u s ed by the enlargement of the affected cells and the large volume of myce-lium present in the intercellular spaces. Finally the affected stem cells are killed and collapse, producing brown sunken areas in the stem. In the young berries, infection is also intercellular, the chlorophyll breaks down and disappears, the cells b e c o me plasmolyzed and col-lapse, and the cell walls turn brown.
At the e n d of the growing season the fungus forms oospores in the infected old leaves, and sometimes in the shoots and berries. T h e oospores remain e m b e d d ed in the leaves even after the leaves fall to the ground and overwinter in them. In the spring, when the leaf tis-sues decay, the oospores are liberated and start ne w infections.
Control
T h e incidence of downy m i l d ew may b e r e d u c ed somewhat by removing or plowing under the leaves at the e n d of the season to re-duce the number of overwintering oospores; also by removing, when practical, the canes near the ground to facilitate free circulation of air and rapid drying of the leaves, and also to reduce the chances of spores b e i ng splashed on the ne w growth in the spring.
Several American grape varieties show considerable resistance to downy mildew, but most E u r o p e an varieties are very susceptible;
even the relatively resistant varieties, however, require protection through chemicals.
T h e most effective fungicides for control of downy mildew are Bor-deaux mixture, ferbam, and captan. T h e applications begin before
bloom and are continued at 7- to 10-day intervals, although the time and number of applications vary with the local conditions, particularly the frequency and duration of rainfall during the growing season.
Selected References
Arens, K. 1929. Physiologische U n t e r s u c h u n g en an Plasmopara viticola, unter b e s o n-derer B e r u c k s i c h t i n g u ng d er I n f e k t i o n s b e d i n g u n g e n. Jahrb. Wiss. Botan. 70:
9 3 - 1 5 7.
Barrett, J. T. 1939. Overwintering m y c e l i um of Plasmopara viticola in the California wild grape, Vitis californica. Phytopathology 29: 8 2 2 - 8 2 3 (abstr.).
D e C a s t e l l a, F., a n d C. C. Brittlebank. 1924. D o w ny m i l d ew of the vine (Plasmopara vi-ticola B. a nd de T.). Dept. Agr., Victoria, Australia, Bull. 4 9 , 4 5 p p.
Gregory, C. T. 1915. S t u d i es on Plasmopara viticola. Intern. Congr. Viticult. Rept. for 1915: 126-150.
Millardet, P. M. A. 1885. (1) T r a i t e m e nt du m i l d i ou e t du rot. J. Agr. Prat. 2: 5 1 3 - 5 1 6 . (2) Traitement du mildiou par le m e l a n ge de sulphate de cuivre et de chaux. Ibid.
p p. 7 0 7 - 7 1 9 . (3) Sur l'histoire du traitement du m i l d i ou par le sulphate de cuivre.
Ibid. p p. 8 0 1 - 8 0 5 . E n g l i sh transl. by F. L. S c h n e i d e r h an in Phytopathol. Classics 3: 1933.
Scribner, F. L. 1886. Report on the fungus d i s e a s es of the grape vine. U.S. Dept. Agr.
Botan. Div. Bull. 2: 123 p p.
Yarwood, C. E. 1937. T h e relation of light to the diurnal cycle of sporulation of certain d o w ny m i l d e w s. J. Agr. Res. 54: 3 6 5 - 3 7 3 .
Zachos, D. G. 1959. R e c h e r c h e s sur la b i o l o g ie et l ' e p i d e m i o l o g ie du mildiou de la v i g ne e n G r e c e. B a s es de previsions et d i v e r t i s s e m e n t s. Ann. Inst. Phytopathol.
Benaki 2: 196-355.
Rhizopus Soft Rot of Fruits and Vegetables
Occurrence and Importance
T h e disease is worldwide in distribution. It affects fleshy organs of vegetable, fruit, and flower crops but is found usually, and is impor-tant only, during storage, transit, and marketing of these products.
Among the crops affected most by this d i s e a se are sweet potatoes, strawberries, all cucurbits, peaches, cherries, peanuts, and several other fruits and vegetables. Corn and some other cereals are affected under fairly high conditions of moisture. Bulbs, corms, and rhizomes of flower crops, e.g., gladiolus and tulips, are also susceptible to this disease. A rotting of the fruit mesocarp (hull) of almond followed by blighting of the adjoining twig and leaves is c a u s ed by the s a me path-ogen.
Rhizopus Soft Rot of Fruits and Vegetables
L o s s es c a u s ed by this d i s e a se vary with the crop, storage tempera-ture and humidity, amount of w o u n d i ng of the fruits or other suscepti-b le plant organs, and amount of inoculum present. Whe n conditions are favorable, the d i s e a se spreads rapidly throughout the containers, and losses can b e very great in a short period of time.
Symptoms
Infected areas of fleshy organs appear water-soaked at first, and they are very soft. If the skin of the infected tissues remains intact, the soft-e n soft-e d flsoft-eshy organ gradually lossoft-es moistursoft-e until it shrivsoft-els into a m u m m y. More frequently, however, the softened skin ruptures during handling or under pressure, e.g., from surrounding fruits, and a whit-ish-yellow liquid drops out. Soon fungus hyphae grow outward through the wounds and cover the affected portions by producing tufts of whiskerlike gray sporangiophores bearing black sporangia at their tips (Fig. 30). T h e bushy growth of the fungus extends to the surface of the healthy portions of affected fruit w h en they b e c o me wet with the exuding liquid and even to the surface of the containers. Affected tis-sues at first give off a mildly pleasant smell, but soon yeasts and bac-teria move in and sour odor develops. Whe n loss of moisture is rapid, infected organs finally dry up and mummify; otherwise they break down and disintegrate in a watery rot.
The Pathogen: Rhizopus sp.
This is a phycomycete found almost everywhere in nature. It lives usually as a saprophyte and sometimes as a weak parasite on stored organs of plants. This fungus is also the cause of the common bread mold. T h e mycelium of the fungus has no cross walls. During its vege-tative growth the fungus produces long, aerial sporangiophores at the tips of which black spherical sporangia d e v e l op (Figs. 30 and 31).
T h e se consist of a thin m e m b r a ne containing thousands of small spherical sporangiospores. Sporangiospores are p r o d u c ed asexually and consist of a cell wall, protoplasm, and nuclei. Whe n the mem-brane of the sporangium is ruptured, the liberated sporangiospores are released and float about in the air or drop to the surface. If they fall on a moist surface or w o u nd of a susceptible plant organ, the sporan-giospores germinate and produce mycelium again. Whe n the myce-lium grows on a surface it produces stolons, i.e., hyphae which arch over the surface and at the next point of contact with the surface pro-d u ce both rootlike hyphae, callepro-d rhizoipro-ds, which grow towarpro-d the
255
F i g. 30. (A) Rhizopus soft rot on s w e et potato. S p o r a n g i o p h o r es a nd sporangia g r o w i ng on the surface of s w e et potato through broken e p i d e r m is a nd in the p r e s e n ce of high relative humidity. (B) Rhizopus soft rot on s q u a s h. (Photo A by courtesy of U.S. D e p t. Agr. Photo  by courtesy of the D e p a r t m e nt of Plant Pathology, Cor-nell University.)
surface, and more aerial sporangiophores bearing sporangia. F r om each point of contact more stolons are p r o d u c ed in all directions.
Whe n environmental conditions, such as temperature, b e c o me unfa-vorable or when the supply of food begins to diminish for the fungus, adjacent hyphae produce short branches called progametangia, which grow toward each other. Whe n they come in contact, the tip of each hypha is separated from the progametangium by a cross wall. T h e terminal cells are the gametangia. T h e se fuse, their protoplasts mix, and their nuclei pair. T h e cell formed by the fusion enlarges and de-velops a thick, black and warty cell wall. This sexually p r o d u c ed spore is called a zygospore. With s o me heterothallic species of Rhizopus, zygospores are produced only when the two gametangia are derived from two sexually compatible strains of the fungus, although the ga-metangia of such two strains show no morphological differences. T h e zygospore is resistant to adverse environmental factors and is the
ov-Fig. 31. Disease cycle of soft rot of fruits and vegetables caused by Rhizopus sp.
Rhizoids UurthriD
Stolon
Sporangiophore Progametangia Hyphae
-Gametangia
Zygote
Zygospore Sexual cycle occurs at the end of growing season or of food supply
Zygospores overwintering Asexual cycle repeated under favorable conditions of temperature and food supply Sporangium Sporangiospores /progressed in soft rot
Zygospores overwintering Asexual cycle repeated under favorable conditions of temperature and food supply Sporangium Sporangiospores /progressed in soft rot