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Controlled environment experiments were conducted to determine the influence of temperature and leaf wetness duration on infection of perennial ryegrass by Rhizoctonia solani. Infection of grass plants raised in pots and exposed to mycelium of R. solani was evaluated at various combinations of temperature and leaf wetness duration. Temperatures included 15, 18, 21, 24, and 27°C. Leaf wetness periods were 9, 12, 15, 18, and 24 h. Disease was most severe (more than 50 leaves with brown patch lesions per pot) when plants were in contact with the inoculum source for 24 h of leaf wetness at 24°C. The least amount of disease (0 leaves with lesions per pot) occurred at 15°C and a 9-h wet period. The data were subjected to analysis of variance with orthogonal polynomial contrasts. Significant effects were included in a regression model that described the response of infection to temperature and wetness duration. The polynomial model included linear and quadratic terms for temperature and wetness duration. The adjusted coefficient of determination for the fitted model was 0.93, indicating an excellent fit to the data. The model is intended for use in an improved brown patch warning system for perennial ryegrass in the midwestern United States.

Controlled environment experiments were conducted to determine the relationship between temperature, leaf wetness duration, and infection of watermelon by Colletotrichum orbiculare. Flats of watermelon seedlings were inoculated and exposed to various combinations of temperature (12, 15, 18, 21, 24, 27, and 30°C) and leaf wetness duration (2, 4, 8, 12, 16, and 24 h). The experimental design was a split-plot, with whole units represented by temperature and subunits represented by leaf wetness duration. Anthracnose incidence, defined as the percentage of symptomatic seedlings in each flat 10 days after inoculation, increased with increasing leaf wetness duration at all levels of temperature. The optimum temperature for infection ranged from 21 to 24°C. At most temperatures, as little as 2 h of leaf wetness was required for infection. Analysis of variance with orthogonal polynomial contrasts and multiple regression procedures was used to define the relationship of anthracnose incidence to temperature and leaf wetness duration.

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Gray leaf spot caused by Magnaporthe oryzae is a serious disease of perennial ryegrass in the midwestern United States. Symptoms of gray leaf spot can be confused with those caused by other fungal diseases that also are common during periods of high temperatures and ample moisture. Because turf managers must select appropriate fungicides for remedial treatment, accurate and timely identification of the pathogen is essential for efficient and effective disease management. We developed and evaluated a polymerase chain reaction (PCR)-based method to detect M. oryzae in infected perennial ryegrass tissue. The method utilizes a commercially available kit that is used for isolation and amplification of plant DNA from leaf tissue. The kit protocol was modified and found to be reliable for the extraction of M. oryzae DNA from infected perennial ryegrass. Primers were designed to amplify a 687-bp fragment of the Pot2 transposon that is found in multiple copies in the genome of the pathogen. The protocol amplified amounts of purified DNA as low as 5 pg and consistently and specifically detected M. oryzae in single diseased leaf blades as well as in field samples of infected perennial ryegrass. The total time required for detection was approximately 4 to 8 h.

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