Symptomless infections in alternaria leaf blight of cotton

Abstract

A symptomless phase of Alternaria macrospora leaf blight that occurs in commercial cultivation of cotton between visible disease outbreaks was studied to determine its cause and effect. Most plants between the two outbreaks of leaf blight epidemics in the field were infected but symptomless; yet during the outbreaks the number of infected symptomless plants was minimal. Drought, high temperature, low humidity, and high salinity increased plant resistance to leaf blight by decreasing visible symptoms and causing symptomless infections. Symptomless plants provided inoculum to infect and produce a visible disease in healthy plants when favorable conditions for disease development resumed. Plant age had minimal influence on symptom-lessness, and sunlight exposure had none. Defoliation was related to the density of spots on leaves and to the virulence of the strain. Polyphenoloxidase activity and phenol accumulation were similar in both disease phases. Plants having visible symptoms produced more spores, but spores from symptomless plants were detached more easily by air currents. Symptom-lessness apparently is a common manifestation of alternaria leaf blight of cotton. It is probably influenced by agroenvironmental parameters that induce the two phases of disease expression. Key words: Alternaria macrospora, cotton leaf blight, leaf spot of cotton, symptomless infections.