Pictorial Assessment Key to Determine Fungicide Concentrations That Control Anthracnose Development on Cucumber Cultivars with Varying Resistance Levels

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Abstract
A pictorial assessment key was developed using photographs of cucumber leaves naturally infected by Colletotrichum lagenarium. The rating scale (1-8) was divided into ranges of 0-1, 1-3, 3-6, 6-12, 12-15, 25-50, 50-75, and 75-100% diseased tissue. The assessment key was used to evaluate anthracnose development at two locations on two cucumber cultivars with genes that condition differing levels of resistance. These cultivars received four rates of chlorothalonil weekly. Higher rates of chlorothalonil application and inoculation of the central four to six plants compared with whole-plot inoculation resulted in less anthracnose development, measured as area under the disease progress curve or final disease severity. The greater resistance to anthracnose in the cultivar Calico compared with that in Calypso was equal to about 0.4 kg of chlorothalonil applied weekly. This fungicide equivalent was not altered by environment but was affected by inoculum distribution and the disease variable used to calculate the fungicide equivalent.

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