University of California

Ecological factors limiting epidemics of hop downy mildew in arid climates


R. M. Sonoda
J. M. Ogawa

Authors Affiliations

R. M. Sonoda was formerly a graduate student in the Department of Plant Pathology, University of California, Davis, is now Assistant Professor and Assistant Plant Pathologist in the Institute of Food and Agricultural Science, University of Florida, Agricultural Research Center, Ft. Pierce, Florida; J. M. Ogawa was Professor of Plant Pathology and Plant Pathologist in the Experiment Station, Davis.

Publication Information

Hilgardia 41(15):457-473. DOI:10.3733/hilg.v41n15p457. December 1972.

PDF of full article, Cite this article


This study reports on factors affecting the development of hop downy mildew during the dry, hot season in the Sacramento Valley, as well as on factors contributing to the paucity of disease development in apparently favorable conditions. In flood-irrigated commercial hop yards, systemically infected shoots (which can be sources of inoculum) were present until late in the growing season. Dew and guttation fluid appeared on leaves and stems at the base of the plants after irrigation, but only a few lesions developed on the leaves.

Data from Hirst spore traps, used to monitor the concentration of sporangia near systemically infected shoots, indicated a diurnal pattern of sporangial release. Sporangia were released with each sudden lowering of RH under laboratory conditions, and a more rapid lowering resulted in greater release. Sporangial release in the field coincided with RH drop. Most of the sporangia were released in the early morning during evaporation of dew and guttation fluid, and few sporangia were collected in the afternoon, evening, or night.

Longevity of viable sporangia was dependent on RH. Relative humidity below 60 per cent killed sporangia in less than 3 hours. Even in well-irrigated yards the ambient RH remained below 60 per cent for about 10 hours each day; most sporangia do not survive throughout the day in such an environment. A few sporangia do survive on hop leaf surfaces through favorable RH conditions, and they initiate infection during the following dew period. Sporangia survived better on the lower surface than on the upper surface because of higher RH.

Sporangia exposed to temperatures of 36, 39, and 42°C had delayed germination when subsequently placed in environments optimum for germination (temperatures of exposed leaves in the yards occasionally exceed 33°C during the day).

The studies indicate that both inoculum and free moisture are present in the yards and can establish new infections, but that they do not occur concurrently long enough to start epidemics. In the event of summer rains, however, even these few infections can provide sufficient inoculum for an epidemic.

Literature Cited

Arens K. Untersuchungen uber Pseudoperonospora humuli (Miy u. Takah.) Wilson, den Erreger der neuen Hopfenkrankheit. Phytopath. Z. 1929. 1:169-93.

Clayton E. C. Cucumber disease investigations on Long Island. New York State Agr. Expt. Sta. Bul. 1928. 558:22

Doran W. L. Downy mildew of cucumbers. Massachusetts Agr. Exp. Sta. Bul. 1932. 283:22

Duvdevani E., Reichert I., Palti J. The development of downy and powdery mildew of cucumbers as related to dew and other environmental factors. Palestine J. Botany, Rehovot Series. 1946. 5:127-51.

Frampton V. L., Longree K. The vapor pressure gradient above a transpiring leaf. Phytopathology. 1941. 31:1040-42.

Hirst J. M. An automatic volumetric spore trap. Ann. Appl. Biol. 1952. 39:257-65. DOI: 10.1111/j.1744-7348.1952.tb00904.x [CrossRef]

Hirst J. M. Changes in atmospheric spore content: diurnal periodicity and the effects of weather. Brit. Mycol. Soc. Trans. 1953. 36:375-93. DOI: 10.1016/S0007-1536(53)80034-3 [CrossRef]

Magie R. O. The epidemiology and control of downy mildew on hops. New York State Agr. Exp. Sta. Tech. Bul. 1942. 267:49

Ogawa J. M., Hall D. H. Epidemiology and control of hop downy mildew in California 1964. p.5. Hop Disease Conference, East Mailing Research Station and Wye College, England. multilith.

Ogawa J. M., Hall D. H., Koepsell P. A., Gregory P. H., Monteith J. L. Spread of pathogens within crops as affected by life-cycle and environment. Airborne Microbes: Seventeenth Symposium of the Society of General Microbiology. 1967. London: Cambridge University Press. p. 247-267.

Osborne T. S., Bacon J. A. Two improved and inexpensive systems for moisture stabilization in seeds and other tissues. Plant Physiol. 1961. 36:309-12. DOI: 10.1104/pp.36.3.309 [CrossRef]

Pinckard J. A. The mechanism of spore dispersal in Peronospora tabacina and certain other downy mildew fungi. Phytopathology. 1942. 32:505-11.

Royle D. J. Diurnal and seasonal fluctuations in spore concentrations of airborne hop pathogens 1968. pp.49-56. Rep. Dep. Hop Res. Wye College for 1967

Royle D. J. Infection periods in relation to the natural development of hop downy mildew (Pseudoperonospora humuli). Ann. Appl. Biol. 1970. 66:281-91. DOI: 10.1111/j.1744-7348.1970.tb06435.x [CrossRef]

Salmon E. S. The downy mildew of hop. The Brewers Jour. 1928. 1928:1-4.

Schnathorst W. C. Relation of microclimates to the development of powdery mildew of lettuce. Phytopathology. 1960. 50:450-54.

Skotland C. B. Infection of hop crown and roots by Pseudoperonospora humuli and its relation to the crown and root rot and overwintering of the pathogen. Phytopathology. 1961. 51:241-44.

Skotland C. B. Factors affecting sporulation and infections by the hop downy mildew fungus, Pseudoperonospora humuli. Phytopathology. 1962. 52:153 (Abstract.)

Sonoda R. M. Ecological studies on the downy mildew of hop 1969. p.183. Ph.D. Thesis. University of California, Davis

Yarwood C. E. Relation of moisture to infection with some downy mildew and rusts. Phytopathology. 1939. 29:933-45.

Yarwood C. E., Hazen W. E. The relative humidity at leaf surfaces. Amer. J. Botany. 1944. 31:129-135. DOI: 10.2307/2437635 [CrossRef]

Zattler F. Uber die Einflusse von Temperatur und Luftfeuchtigkeit auf Keimung und Fruktifikation von Pseudoperonospora humuli und auf das Zustandekommen der Infektion des Hopfens. Phytopath. Z. 1931. 3:281-302.

Sonoda R, Ogawa J. 1972. Ecological factors limiting epidemics of hop downy mildew in arid climates. Hilgardia 41(15):457-473. DOI:10.3733/hilg.v41n15p457
Webmaster Email: wsuckow@ucanr.edu