The incidence and severity of tomato blight disease are typically assessed through visual observation and inspection of tomato plants. Plant pathologists and agricultural experts examine the leaves, stems, and fruits of tomato plants for symptoms such as lesions, discoloration, wilting, and overall plant health. They may also use various diagnostic tools such as laboratory tests to confirm the presence of specific pathogens responsible for the disease.

Joseph Andrew Wandulu

Look for reference Dorrance, A. E., and Inglis, D. A. 1997. Assessment of greenhouse and laboratory screening methods for evaluating potato foliage for resistance to late blight. Plant Dis. 81:1206-1213.

According to: Desta, M., & Yesuf, M. (2015). Efficacy and economics of fungicides and their application schedule for early blight (Alternaria solani) management and yield of tomato at South Tigray Ethiopia. Journal of Plant Pathology and Microbiology, 6(268), 20, in general, disease incidence and severity recorded every week starting from the first appearance of disease symptoms. Incidence of disease is assessed by counting the number of infected plants and expressed as percentage of total plants assessed. Severity of early blight is recorded on the basis of 1-6 rating scales (up to 20% leaf infection, 21-41%, 41-60%, 61-80%, 81-99%, and 6=100% leaf infection or plant defoliation).

Different scales are applied for late blight, including simplified and broad keys, respectively, with six (3, 12, 22, 40, 60 and 77% or 1, 5, 10, 16, 32 and 50%)) and eight (3, 6, 12, 22, 40, 60, 77 and 90%) levels of disease severity, .

Correa, F. M., Bueno Filho, J. S., & Carmo, M. G. (2009). Comparison of three diagrammatic keys for the quantification of late blight in tomato leaves. Plant Pathology, 58(6), 1128-1133.