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

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Abstract
The relationships between the net photosynthetic rate of bean leaves and severity of rust, angular leaf spot and anthracnose were quantified at different temperatures of plant incubation, stages of disease development, and phenological stages of the crop, in two bean cultivars. Several experiments were performed in controlled environment chambers and in the field. The virtual lesion concept was used to quantify the reduction in photosynthetic efficiency of the green leaf tissue surrounding the lesions. The β parameter that expressed the ratio between areas of the virtual and visual lesions was estimated according to the model Px /P0 = (1 − x)β, where Px was the net photosynthetic rate of a leaf with severity x and P0 was the average net photosynthetic rate of healthy leaves. The β‐value was 2·17 ± 0·02 (R2 = 0·88) for rust, 3·81 ± 0·04 (R2 = 0·91) for angular leaf spot, and 7·97 ± 0·13 (R2 = 0·94) for anthracnose. For each disease, the parameter β was consistent regardless of incubation temperature, stage of disease development, bean phenological stage and bean cultivar. In addition, the relationships between bean yield and the variables of area under the disease progress curve (AUDPC), healthy leaf area index duration (HAD) and healthy leaf area index absorption (HAA), published in the literature for rust, angular leaf spot and anthracnose, were recalculated with the virtual disease severity. Bean yield was more closely related to the new variables ‘photosynthesizing leaf area index duration’ (PAD) and ‘photosynthesizing leaf area index absorption’ (PAA) than to those variables previously published, but only for diseases with a large β‐value and at high levels of disease severity.