The combined effects of temperature, leaf wetness, and inoculum concentration on infection of cucumbers with Pseudoperonospora cubensis

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Abstract
The infection of susceptible cucumber leaves by Pseudoperonospora cubensis was investigated under controlled conditions in a 6 × 3 × 4 factorial experiment with 72 combinations of temperature, inoculum concentration, and leaf-wetness duration. The minimum, maximum, and optimum levels of each factor for symptom production were found to depend on one or both of the other factors. The minimal temperature for infection was 20 °C with 2 h of wetness and this decreased to 10–15 and 5–10 °C with 6 and 12 h of wetness, respectively, according to the inoculum load. The maximal temperature for infection was 25 °C, except for a single treatment in which some infection occurred at 28 °C. Optimal temperatures for infection were 20, 15–20, 10–20, and 5–20 °C at dew periods of 2, 6, 12, and 24 h, respectively. The shortest dew period needed to establish infection was 2 h. A sharp increase in infection was associated with extending the dew period to 6h. Further lengthening of the dew period resulted in more infection under unfavorable combinations of temperature and inoculum concentration. An inoculum load of 10 sporangia/cm2 was minimal for infection under favorable combinations of temperature and wetness, and higher loads were needed under unfavorable combinations. The optimal inoculum concentration was 1000 sporangia/cm2 under most inoculation circumstances. The implications of the variable behaviour of the pathogen to disease development in the field are discussed.

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