Cultural, morphological, pathogenic and molecular variability amongst tomato isolates of Alternaria solani in India

Abstract

Early blight (Alternaria solani) is an important disease causing severe damage in tomato. The eleven isolates of A. solani designated as So, Dh, Sh, Va-5, Ka, Ma, Hy, Ba-1, My, Va-3 and Mi were collected from different agroclimatic conditions and these isolates were characterized for cultural, morphological, pathogenic and molecular variations. The pigmentation varied from yellow, brown, black, brownish to greenish black in isolates of A. solani on potato dextrose agar medium. In general, radial growth of all isolates ranged between 14.9 mm and 32.2 mm on PDA and 24.3 mm to 53.7 mm on three selective media i.e., ASM, V-8 juice agar and V-8 juice agar (synthetic) on the fourth day. The fastest radial growth was recorded in the So isolate and slowest in the Ka isolate on PDA, while isolates Dh, Ba-1 and Va-3 were recorded to be faster in growth on ASM, V-8 juice agar and V-8 juice agar (synthetic) medium. The thickness of conidiogenous hyphae varied between 1.17 μ and 9.56 μ, with maximum in the Va-5 and Ma isolates. Most of the isolates showed smooth mycelial growth with circular and irregular margin and without concentric zonation. Sporulation was not found in any of the isolates on four different nutrient media, whereas conidiogenous hyphal length was observed in V-8 juice agar medium only. Based on the pathogenicity, isolates of A. solani were rated as virulent or less virulent based on percentage disease incidence data. Molecular variability studies were also done to find out the best annealing temperature and eighty-six primers were screened to select for maximum polymorphism of DNA. The best annealing temperature was recorded between 32.5 °C and 34.0 °C for the pathogen, and most efficient amplification and polymorphism of DNA was found with random primer 5′-CGCGTTCCTG-3′.