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Growth of `Earligold' muskmelon (Cucumis melo L.), expressed as plant dry weight from transplanting to anthesis, could be predicted using a multiple linear regression based on air and soil temperatures for 11 mulch and rowcover combinations. The two independent variables of the regression model consisted of a heat unit formula for air temperatures, with a base temperature of 14C and a maximum reduced threshold of 40C, and a standard growing-degree day formula for soil temperatures with a base temperature of 12C. Based on 2 years of data, 86.5% of the variation in the dry weight (on a log scale) could be predicted with this model. The base temperature for predicting developmental time to anthesis of perfect flowers was established at 6.8C and the thermal time ranged between 335 and 391 degree days in the 2 years of the experiment.

Late leaf spot, induced by Cercosporidium personatum (Berk. & Curt.) Deighton, is the most important foliar disease affecting peanut (Arachis hypogaea L.) in Florida and neighboring states. The disease first occurs as necrotic lesions on peanut leaflets and subsequently induces leaflet abscission. Field experiments were conducted to study the effects of late leaf spot on leaflet and canopy photosynthesis. Fully expanded leaflets at the tip of the main stem were tagged and leaflet photosynthesis was measured during the subsequent weeks with a portable photosynthesis system. Leaflet photosynthesis was reduced linearly with the increase in the percentage of necrotic leaf area. Reduction in light utilization due only to necrotic leaf area did not explain completely the reduction in leaflet photosynthesis. Regression analysis suggested that 15% necrotic leaf area contributed to a 65% reduction in the photosynthesis of infected leaflets. It appears that there was an effect on leaflet photosynthesis due to host cells invaded or affected by the pathogen. A chamber made of aluminum frame covered with mylar film was used to estimate canopy photosynthesis over a land area of 0.56 m2. At the canopy level, the effect of reduction in leaflet photosynthesis was negligible compared to the effect of disease‐induced defoliation. The decrease in leaf area index was the major component involved in the reduction of canopy photosynthesis because of the effect of late leaf spot. Canopy photosynthesis was inversely proportional to total disease severity, which is an expression of both defoliation and necrotic area.

In controlled environment studies, the influence of temperature and wetness duration on infection of strawberry leaves by Mycosphaerella fragariae was quantified by inoculating plants with a conidial suspension and incubating them at various combinations of temperature (5 to 35 degrees C) and leaf wetness duration (0 to 96 h). Infection was expressed as the number of lesions per square centimeter of leaf surface and relative infection was used to develop an infection model. Younger leaves were more susceptible to infection. Regardless of temperature and duration of leaf wetness, only few lesions developed on the oldest (19 to 21 days old) and intermediate leaves (12 to 15 days old), respectively (maximum of 1.7 and 2.3 lesions per cm(2)) as compared to the youngest leaves (5 to 7 days old; maximum of 12.6 lesions per cm(2)). On the youngest leaves, lesions developed at all temperatures except at 35 degrees C, and the number of lesions, for all leaf wetness durations, increased gradually from 5 to 25 degrees C and decreased sharply from 25 to 30 degrees C. For temperatures of 15 and 20 degrees C, the number of lesions increased gradually when leaf wetness duration increased from 12 to 96 h. At 25 degrees C, the number of lesions increased with increasing leaf wetness from 12 to 48 h and then at a higher rate from 48 to 96 h. The optimal temperature for infection was 25 degrees C. For most temperatures, a minimum of 12 h of leaf wetness was necessary for infection (more than 1 lesion per cm(2)). Relative infection was modeled as a function of both temperature and wetness duration using a modified version of the Weibull equation (R (2) = 0.98). The resulting equations provided a precise description of the response of M. fragariae to temperature. The model was sufficiently flexible to account for most characteristics of the response of M. fragariae to wetness duration. The model was used to construct a risk chart that can be used to estimate the potential risk for infection based on observed or forecasted temperature and leaf wetness duration.