NC State and USDA Cucumber Disease Handbook
Downy Mildew (Pseudoperonospora cubensis)
Pseudoperonospora cubensis (Berk. and Curt.) Rostow.
Description on Host:
P. cubensis is an obligate parasite, causing yellow spots on the upper sides of cucumber leaves with corresponding fluffy purplish mildew on the lower sides.
Mycelium: Coenocytic and intercellular in host tissue; haustoria small ovate and intracellular. Conidiophores arise in groups of one to five through the stomata; either dichotomous or intermediate between dichotomous and monopodial branching habit. Spores: Sporangia produced singly on tips of sporangiophore branches; grayish to olivaceous purple; avoid to ellipsoidal (14-23 x 21-39 microns); thin walled, with a papilla at the distal end. Sporangia germinate by formation of flagellate zoospores and asexual conidia. Oospores reported from USSR, China and Japan.
P.H. Williams; Department of Plant Pathology; University of Wisconsin; 1630 Linden Drive; Madison, WI 53706.
Biotypes of biological races are known. Pathogenic stability in nature not studied.
Storage and Retrieval:
The fungus can be maintained at 16°C in infested cucumber seedlings for 3 to 4 weeks after inoculation. Sporulating cotyledons, frozen in plastic bags, remain pathogenic at least 10 weeks, but viability is decreased (may result in loss of pathogen). See inoculum increase section for inducing sporulation on cotyledons.
Fungus maintained in susceptible cucumber seedlings. To increase inoculum, inoculate cotyledons of Wisconsin SMR 18 grown in plastic freezer cups (9.5 x 9.5 x 7.5 cm), 12 plants/cup, as for routine inoculation (see inoculum preparation, quantification, and inoculum distribution and delivery sections). Incubate in the dark for 48 hr at 20°C and 100% relative humidity (RH). Subsequently grow plants at 16°C. Fertilize with Hoagland’s solution 3 times/wk.
Induce sporulation on infected seedlings 2-3 weeks after inoculation by misting and placing in the dark for 24 hr at 20°C and 100% RH. Necrotic tissue does not produce sporangia. Remove cotyledons and dip in distilled water to wash off plant exudates. Place cotyledons in a beaker of distilled water and rub cotyledons with the fingers to dislodge the sporangia.
Count sporangia with hemacytometer. Adjust spore concentration to 12 x 10E4 sporangia/ml with distilled water.
Inoculum Distribution and Delivery:
Using a Pasteur pipette place a .01-.03 ml droplet of inoculum on the upper surface of the cotyledon.
Cucumis sativus L., cucumber.
Source of Resistance:
PIs and cultivars reported to contain some resistant plants PI 197807, Puerto Rico 37, Puerto Rico 40, Bangalore, and China.
Differentials – Controls:
Susceptible check Wisconsin SMR 18. Resistant check Gy 14.
Growth of Host:
Cucumber seeds are sown in steam sterilized coarse grade vermiculite in wooden flats (52 x 36 x 7 cm). Each flat contains 10 rows, 25 seeds/row. Resistant and susceptible checks are sown in row 6. The flats are placed on a heated germination bench. Vermiculite temperatures of 32°C insure rapid and uniform germination. Newspaper on top of the flat prevents cooling by evaporation. Newspaper is removed when seeds germinate.
Inoculate cotyledons 3-4 days after emergence when cotyledons are expanded.
Incubate plants in the dark for 48 hr at 20°C and 100% RH. Subsequently place plants in greenhouse 24-28°C.
Plants are rated as susceptible or resistant 7-8 days after inoculation. Susceptible plants have strong chlorosis, while resistant plants have no chlorosis or very faint localized chlorosis.
Simultaneous inoculation with scab, anthracnose, angular leaf spot, or bacterial wilt. Subsequent inoculation with cucumber mosaic virus (CMV) and powdery mildew.
Both resistant and susceptible plants can be transplanted to steam sterilized soil.
Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin