Variability of Alternaria isolates causing leaf blight diseases in cucurbits

Abstract

Alternaria leaf blight is one of the serious diseases of cucurbits in Kerala. A study on “Variability of Alternaria isolates causing leaf blight diseases in cucurbits” were conducted at College of Horticulture, Vellanikkara during 2004 - 2005 and they include symptomatology, cultural, morphological, physiological and nutritional characters, host range and effect of toxin on disease development and also on the variability if any existing among the different isolates of Alternaria.

Isolation of Alternaria from diseased samples of ash gourd, snake gourd, ridge gourd, bottle gourd, bitter gourd, ivy gourd and pumpkin collected from Vellanikkara and Elanad areas of Thrissur district was done in Potato dextrose agar medium and isolation of diseased specimens yielded fungal growth similar to Alternaria sp. and obtained seven isolates from Vellanikkara and six isolates from Elanad. Symptom or infection usually started from leaf margins as small, irregular brown lesions. Later brown lesions enlarged in size with characteristic yellow halo. Variation in symptom expression was noticed only with respect to the presence or absence of concentric zonations. Ash gourd, bitter gourd and ivy gourd produced symptoms with concentric zonations on the infected area.

Variation in cultural and morphological characters of different isolates of Alternaria was studied. Alternaria spp. produced thick and velvety colony with brownish grey to dark brown aerial mycelium and the isolates showed variation in colony character only with respect to the presence of concentric zonations in the culture. All isolates sporulated well on Potato dextrose agar medium and spore count ranged from 2.5 to 4.9 x 105 spores ml-1. Ridge gourd isolate from Vellanikkara produced maximum spores and minimum spores were produced by the isolate of ivy gourd from Elanad. Variation in growth rate was observed among different isolates of Alternaria grown on different media. Among the different media tested Potato dextrose agar supported maximum growth of different isolates. All the isolates were found to produce conidiophores and conidia in pure culture. Variations in the size and branching of conidial chain were also noticed and size of conidial chain varies from 2-12.

Cultural and morphological characters revealed that Alternaria alternata (Fr.) Keissler and Alternaria tenuissima (Kunze ex Pers.) Wiltshire was the two main species of Alternaria causing leaf blight diseases in various cucurbitaceous vegetables. There was no earlier reports of these species of Alternaria on ash gourd, snake gourd, bottle gourd and pumpkin and hence it is the first report of Alternaria alternata (Fr.) Keissler on ash gourd, snake gourd, bottle gourd and pumpkin and also Alternaria tenuissima (Kunze ex Pers.) Wiltshire on snake gourd as the pathogen of leaf blight diseases from India.

Cluster analysis of morphological characteristics revealed a degree of variability among the isolates. The lowest dissimilarity index was noticed between A. alternata isolates of ash gourd and snake gourd of Vellanikkara. A. tenuissima of bitter gourd and ivy gourd from Vellanikkara found more similar in their morphological characters. A. tenuissima of snake gourd and bitter gourd of Elanad area stands separate and the latter showed maximum dissimilarity with other isolates. An in vitro study on the effect of physiological and nutritional characters such as temperature, pH, light, carbon and nitrogen sources on different isolates of Alternaria were carried out and observed that, among the different temperatures tested, 25 to 30oC was found to be the most favourable temperature range for the growth of different Alternaria isolates and the pH levels 6 and 7 supported the growth of most of the isolates. Among the light treatments full light and alternate light and dark were found to be better for the growth, medium containing starch as the carbon source and KNO3 and MgNO3 as nitrogen source supported maximum growth of different isolates of Alternaria.

Study on conidial germination revealed that all isolates took three hours for starting germination and 10 h for cent per cent germination. All cells of conidium germinated by producing germ tube which elongated to form hyphae. All isolates of Alternaria species obtained from different cucurbitaceous vegetables from both the locations were found to be compatible with each other when grown together.

Histopathological changes brought about by all the isolates of Alternaria were almost similar. Due to infection epidermal cells get destructed and hyphae and conidiophores of the fungus seen emerging through the broken epidermis. Destruction of chloroplast and necrosis and death of complete cells occurred in the final stages. Host range study proved that all the isolates of Alternaria from both the locations produced symptoms on all the cross inoculated cucurbits. They were also found producing symptoms on solanaceous vegetables and other vegetables viz., amaranthus, cowpea and bhindi. From the toxin study, it was found that both the exotoxin and endotoxin were effective in producing symptoms on all cucurbitaceous, solanaceous and other vegetables tested. The exotoxin produced the symptoms first.