Accounting for photosynthetic efficiency of bean leaves with rust, angularleaf spot and anthracnose to assess crop damage

The relationships between the net photosynthetic rate of bean leaves and se verity of rust, angular leaf spot and anthracnose were quantified at differ ent temperatures of plant incubation, stages of disease development, and ph enological stages of the crop, in two bean cultivars. Several experiments w ere performed in controlled environment chambers and in the field. The virt ual lesion concept was used to quantify the reduction in photosynthetic eff iciency of the green leaf tissue surrounding the lesions. The beta paramete r that expressed the ratio between areas of the virtual and visual lesions was estimated according to the model P-x /P-0 = (1 - x)(beta), where P-x wa s the net photosynthetic rate of a leaf with severity x and P-0 was the ave rage net photosynthetic rate of healthy leaves. The beta -value was 2.17 +/ - 0.02 (R-2 = 0.88) for rust, 3.81 +/- 0.04 (R-2 = 0.91) for angular leaf s pot, and 7.97 +/- 0.13 (R-2 = 0.94) for anthracnose. For each disease, the parameter beta was consistent regardless of incubation temperature, stage o f disease development, bean phenological stage and bean cultivar. In additi on, the relationships between bean yield and the variables of area under th e disease progress curve (AUDPC), healthy leaf area index duration (HAD) an d healthy leaf area index absorption (HAA), published in the literature for rust, angular leaf spot and anthracnose, were recalculated with the virtua l disease severity. Bean yield was more closely related to the new variable s 'photosynthesizing leaf area index duration' (PAD) and 'photosynthesizing leaf area index absorption' (PAA) than to those variables previously publi shed, but only for diseases with a large beta -value and at high levels of disease severity.