Diseases

Pseudoperonospora cubensis Rostow. - Downy Mildew of Cucumbers

Systematic position.

Division Oomycota, order Peronosporales, family Peronosporaceae, genus Pseudoperonospora.

Biological group.

Obligate parasite.

Morphology and biology.

The disease appears on the upper side of leaves of cucumbers (less often on melons, pumpkins, water-melons), yellow in the beginning, with brown spots located along veins showing up later. Spots grow in size and often coalesce. The affected tissue of leaves dries up, becoming fragile, and the leaves fall. With high humidity, especially in protected ground, the disease causes rotting of leaves. Dense grayish-violet bloom is formed on lower side of leaves in spots. The bloom consists of ramified mycelium bearing ovoid, pyriform, or palmate haustoria. Zoosporangiophores are located in bunches of 2-7, less often singly, leaving the leaf through broken cuticle, 150-25 x 7-9 microns, with apical branchlets positioned under right angles. Zoosporangia are ellipsoid, ovoid, with papillar tubercle at apex, grayish or violet, sometimes brown, 20-28 x 16-20 microns. Wintering stage of the fungus is an oospore; the oospores are spherical, yellowish, with rough coat, 36-42 microns in diameter.

Distribution.

Downy Mildew of Cucumbers was found for the first time in Russia by S.I. Rostovtsev in 1903 in the Tver Region. Now the disease is widely distributed in all regions of the country where cucumber is cultivated on open and protected ground.

Ecology.

Oospores on residues of affected plants and in seeds, are a source of the infection. Optimum temperature for fungus development is 15-22.C at relative air humidity 80-100%, the incubation period is 3-6 days. The disease development is promoted by rains, dews, downturn of temperature, and also by the cultivation of cucumbers on badly aired and shady plots, or in hothouses having insufficient solar light exposure and excessive humidity.

Economic significance.

Downy Mildew causes the infection and fast death of plant leaves, abscission of ovaries, and yellowing and withering of fruits. As a result, the yield is reduced by 30 to 100%. Control measures include cleaning and destruction of vegetation residues, deep autumn plowing, the maintenance of crop rotation (with return of gourds on a former field in 2-3 years), disinfection of soil, hotbeds and hothouses, seed dressing before sowing, preventive fungicide spraying of plants during vegetation and the introduction of resistant varieties.

Reference citations:

Efimov M.S., Sklyarevskaya V.V., Ol.khovskaya N.Ya. 1978. Downy Mildew in Ukraine. Zashchita rastenii 12: 37. (In Russian)
Gorokhovskii V.F. 2002. Method of estimation of affection of cucumber by Downy Mildew. Selektsiya i semenovodstvo 1: 27-28. (In Russian)
Grin.ko N.N. 2003. Downy Mildew of Cucumber. Sochi. 68 p. (In Russian)
Grin.ko N.N., Zherdetskaya T.N. 1992. Bioecological features of activator of Downy Mildew of Cucumber. In: Samersova V.F., ed. Protection of plants. Collection of works of Bel. NIIZR. V. 17. Minsk: Bel.NIIZR. 52-61 p. (In Russian)
Hawksworth D.L., Kirk P.M., Sutton B.C., Pegler D.M. 1995. Ainsworth & Bisby.s Dictionary of the fungi. Kew: CAB International. 616 p.
Kuznetsov N.N. 1984. Diseases of cucumber in hothouses and bases of its control in the Far East. PhD Thesis. Leningrad: MSKH SSSR, LSKHI. 18 p. (In Russian)
Levitin M.M. 1967. Downy Mildew of Cucumbers. Kartofel. i ovoshchi 3: 36-38. (In Russian)
Nalobova V.L. 1986. Gray Mould Blight and Downy Mildew of Cucumbers in protected ground on the territory of Byelorussia. In: Yurkevich I.D., ed. Botany (researches). V. 27. Minsk: Nauka i tekhnika. 189-191 p. (In Russian)
Osnitskaya E.A. 1970. Diseases of vegetables. In: Polyakov I.Ya., Minkevich I.I., eds. Pests and diseases distribution on agricultural crops in RSFSR in 1969 and the forecast of their appearance in 1970. Moscow: MSKH RSFSR, VIZR. 252-263 p. (In Russian)
Osnitskaya E.A. 1971. Diseases of vegetables. In: Polyakov I.Ya., Minkevich I.I., eds. Pests and diseases distribution on agricultural crops in RSFSR in 1970 and the forecast of their appearance in 1971. Moscow: MSKH RSFSR, VIZR. 187-198 p. (In Russian)
Papoyan F.A., Kostanyan A.V. 1975. Downy Mildew of Cucumbers in Ararat Valley. In: Khodzhoyan K.M., ed. News of agricultural sciences. N. 10. Erevan: Min. SKH Arm. SSR. 61-62 p. (In Russian)
Ragimov U.A. 1961. Downy Mildew of Cucumbers. Protection of plants against pests and diseases. 35 p. (In Russian)
Zherdetskaya T.N. 2000. Severity of Downy Mildew of Cucumber in Byelorussia. In: Soroka S.V., ed. Protection of plants. Collection of works of Bel. NIIZR. V. 19/23 KHIX/XXIII. Minsk: Bel. NIIZR. 118-124 p. (In Russian)
Zolotareva E.V., Migina O.N. 2001. Features pathogenesis of the activator of Downy Mildew of Cucumber in Amur Region. In: Romanova T.A., ed. Works. Agriculture. Animal husbandry. V. 2. Khabarovsk: Rossel.khozakademiya, Dal.. NIISKH. 27-32 p. (In Russian)

© Khlopunova L.B.