MARKET DISEASES OF FRUITS AND VEGETABLES 17 



of the harvested product, will do much to reduce losses from this 

 decay. 



(See 32, 66, 85, 116, 117, 1M.) 



Downy Mildew 



(Peronospora spinaciae Laub.) 

 OCCURRENCE, SYMPTOMS, AND EFFECTS 



Downy mildew is found throughout all the commercial spinach- 

 growing areas of the country. Field losses of significance occur regu- 

 larly and during some seasons may be very extensive. On the market 

 it constitutes one of the most prevalent and important diseases of 

 spinach. The leaf blemishes detract from the appearance and quality 

 of the marketed product and, when in an advanced stage, frequently 

 afford ready infection courts for secondary soft rot bacteria. 



The appearance of pale-yellow areas in the normal green of the leaf 

 usually constitutes the first symptom of the disease. These areas 

 or spots are irregular in shape, are without clearly defined margins, 

 and vary considerably in size (pi. 9, A). They are apparent on both 

 upper and lower leaf surfaces. Under humid conditions the pathogen 

 develops over the surface of the infected areas on the lower side of 

 the leaf as a downy gray mold that soon turns lilac gray or purple 

 gray (pi. 9, B). Symptoms vary with humidity. Thus, under very 

 low relative humidity yellowing may occur without any signs of the 

 pathogen appearing, while under high humidity the downy mildew 

 may develop before there is any evidence of yellowing. Under very 

 humid conditions the pathogen may occasionally be found fruiting 

 on the upper leaf surface and the petioles. Lower leaves are fre- 

 quently the first to be infected. Severely infected leaves may dry 

 up or under wet conditions become water-soaked and brownish and 

 soon decay. 



There is some evidence that new spots may appear and old ones 

 become more prominent during the transit and marketing periods. 



CAUSAL FACTORS 



The pathogen, Peronospora spinadae, is one of the downy mildew 

 fungi and can grow and reproduce only while associated with living 

 tissue. Only spinach is attacked. The fungus reproduces primarily 

 by means of the conidia, or summer spores, which form in great abun- 

 dance under humid conditions (relative humidity of 85 percent or 

 above). It is the presence of conidia and of the conidiophores on 

 which they are borne that gives the mildew the purplish-gray color. 



The conidia are splashed from leaf to leaf by rain and are carried 

 from plant to plant by winds, insects, or other agencies. The pres- 

 ence of surface moisture is necessary for germination of the conidia. 

 The temperature range through which infection may occur is 40° to 

 75° F. with an optimum temperature for germination of the conidia 

 of about 48°. At temperatures of 60° to 65° infection of spinach leaves 

 may occur within 3 hours after they are inoculated. Fruiting of the 

 fungus on these new lesions may take place 6 days after inoculation. 



579410 °— 44 2 ' ~ 



