Characteristics and pathogenicity of Pythium spp. associated with root rot of tomatoes in soilless culture in Brittany, France

Pythium species cause necrotic lesions and root rots on tomato in soilless culture. Epidemiological studies conducted in Brittany, France, revealed the precocity and the frequency of the contamination of tomato roots by these fungi, even in symptomless roots. The colonization, estimated by culture-plate methods, could reach levels as high as 10 5 propagules/gram of root. The frequent occurrence of Pythium with filamentous non-inflated sporangia, representing 75% of the total isolates, was observed during both latent infections and necrotic phases. Those Pythium spp., grouped as Pythium F, induced variable necrotic lesions in tomato roots in soilless culture. This group, which had been overlooked previously in favour ofknown pathogenic species such as P. ultimum var. ultimum, can be pathogenic to tomato plants and therefore form a component of a potential disease complex.