36 MISC. PUBLICATION 4 4 0, TJ. S. DEPARTMENT OF AGRICULTURE 



DOWNY MILDEW 

 (Peronospora viciae (Berk.) DBy. ) 



Although downy mildew may occur in practically any part of the 

 country in which peas are grown, it is seldom of economic importance 

 except in the humid regions along the coast of California, Oregon, and 

 Washington. Both field and garden varieties of peas are susceptible 

 to this disease. 



The first symptoms of downy mildew appear as small yellowish 

 spots on the upper surface of the leaflets. As these areas enlarge, a 

 whitish-gray or drab-colored mildew may be observed on the under 

 side of affected leaflets. This mildew is made up of the mycelium and 

 numerous spores of the fungus. If severely infected a whole leaflet 

 may shrivel, turn brown, and die. Occasionally, affected leaA^es show 

 grayish-white porcelaneous areas, which indicate that the fungus is 

 producing resting spores (oospores) within the tissues. Frequently 

 there is also severe infection of the pods, and if these are not dis- 

 carded before packing for shipment, the lesions are considered a 

 serious blemish on the market. 8 Pod infections show as yellowish 

 slightly raised irregular blotches, but there is no surface mold visible 

 (pi. 10, C). The internal tissues of the pod wall in these yellowish 

 blisterlike areas are filled with oospores. These blotches do not be- 

 come soft or decay unless some other organism invades the tissues. In 

 affected pods the seeds underneath the yellow blotches on the pod 

 wall are usually aborted or undersized and may show discoloration. 

 The inside of the wall of affected pods usually shows white velvety 

 proliferations under the yellow areas (pi. 10, D). These white velvety 

 patches have the appearance of a heavy coat of mold, but in most 

 instances no mycelium is visible in them. Other injuries and diseases 

 sometimes stimulate the pod wall to develop similar white patches. 

 (See Scab, p. 39.) 



Downy mildew does not cause serious damage to peas, except in cool 

 regions where fogs and rains keep the foliage moist. The spores of 

 the causal fungus are spread by winds and splashing water. They can 

 germinate and cause new infections overnight at temperatures between 

 50° and 70° F. It has been shown experimentally that some infection 

 may take place even at 32°, but the most active growth is made at near 

 60°, and little or no development occurs at 80°. 



If climatic conditions favor the development of the causal organ- 

 ism, the control of downy mildew in peas is very difficult. It is 

 thought that the fungus is carried over from year to year as perennial 

 mycelium and oospores in decaying vines and leaves thai become 

 mixed into the soil or is reintroduced by planting seed infected with 

 the organism. Seed treatments or the spraying of plants is usually 

 not very effective. Pea seed grown in semiarid regions i> likely to he 

 free from downy mildew. A 2-year rotation of crops has been sug- 

 gested as one method of holding this disease in cheek in some localities. 



(See 78, !Jp5.) 



'Ramsey, G. I'., oospore stage of peronospora viciab on peas. T t . S. Bur. Plant Indus. 

 I'l.ini Dis. Rptr. 15 : 52-53. 1931. [Mimeographed.] 



