Localized hemibiotrophy in Colletotrichum: cytological and molecular taxonomic similarities among C. destructivum, C. linicola and C. truncatum

The infection process of hemibiotrophic isolates of Colletotrichum linicola (from flax, Linum usitatissimum) and C. truncatum (from broad bean, Vicia faba and lentil, Lens culinaris) was studied by light microscopy. Host surfaces were penetrated directly leading to a symptomless, biotrophic phase characterized by the elaboration of large multilobed, multiseptate, vesicular primary hyphae that were restricted to the initially infected epidermal cells. Biotrophy lasted for the first 48 h of the host-pathogen interaction and was rapidly succeeded by a necrotrophic phase during which narrow, secondary hyphae invaded the surrounding leaf tissues and water-soaked spreading lesions with sporulating, monosetate acervuli were produced on infected host surfaces. Molecular taxonomic analysis of the nucleotide sequences of the amplified D2 and ITS-2 regions of rDNA revealed very close similarities (97-99%) between these isolates and those of C. destructivum obtained from cowpea (Vigna unguiculata) and lucerne (Medicago sativa), and also of C. truncatum obtained from pea (Pisum sativum). This association was consistent with results from a comparative assessment of some in-planta and in-vitro morphological and growth characteristics of these hemibiotrophic fungi. It was concluded that localized hemibiotrophy is an infection strategy utilized predominantly by a closely-related group of pathogens comprising C. destructivum, C. linicola and C. truncatum, and the formation of multilobed primary hyphae restricted to the first penetrated cell might therefore be a key taxonomic character which correlates consistently with ITS sequence data.