Abstract
To discover characters for distinguishing pathotypes, microscopical examinations were made of 1797 cysts of twelve Heterodera rostochiensis Woll. and H. pallida Stone isolates of known pathotype. The cysts, reared on two host plants - resistant Lycopersicon peruvianum B6001 and susceptible L. esculentum 'Ailsa Craig' - could be placed in two groups on the basis of cyst-fenestral length, width, area, and shape, and on the basis of Granek's ratio and B (the distance between the anal pore and the nearest fenestral margin). The groups could be related to the predominant species present in the isolate. Cysts of H. rostochiensis (pathotype A) tend to have a small, circular vulval fenestra; those of H. pallida tend to be oval and larger; cysts with fenestral length < 19μm are likely to be H. rostochiensis. Qualitative cyst characters - pigmentation, punctation, perineal pattern, fenestralia, thornlets and microbullae were no help in distinguishing pathotypes. Comparison of the results with those of other studies on round-cyst Heteroderas shows that Granek's ratio and perineal cyst-wall pattern are unreliable for species differentiation.
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