Peanut Web Blotch: II Symptoms and Host Range of Pathogen1

Abstract

Abstract Web blotch, caused by Phoma arachidicola, has varied in severity from year to year in Texas, depending on survival of inoculum on peanut crop residue, presence of susceptible peanut cultivars and suitable environmental conditions, especially favorable temperature and high relative humidity. The web and blotch symptoms may develop in sequence on the same leaflet or independently on different leaflets. At relative humidities below 80% within the plant canopy, the fungal hyphae grew under the leaf cuticle causing a web-type symptom. At relative humidities above 90%, hyphal strands grew more repidly and branched extensively in the leaf resulting in the blotch-type symptom. P. arachidicola isolates from South Africa, Argentina, and the USA were all pathogenic and caused similar symptoms. Of 32 legumes, peanut, sweetclover, hairy vetch and alfalfa were the most susceptible. Symptoms on hosts other than peanuts consisted of small necrotic spots. A hypersensitive reaction developed on eight legumes. Typical web blotch symptoms were observed only on peanuts in the field.