Leaf Spots Induced by Ascochyta pisi and Mycosphaerella pinodes

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Abstract
The leaf-spot pathogens, Ascochyta pisi and Mycosphaerella pinodes, both cause limited, necrotic lesions in detached pea leaflets suspended above water. When leaflets were floated on water A. pisi lesions were still limited, but those caused by M. pinodes spread rapidly to occupy all the leaflet tissue. Increasing the numbers of spores in inocula decreased numbers of mature lesions caused by A. pisi, but increased numbers of spreading lesions caused by M. pinodes. Older leaflets were more susceptible to both pathogens. Studies of penetration and colonization of leaves with the aid of light and electron microscopy showed that cell-wall-degrading enzymes were involved in the formation of A. pisi lesions and in spreading lesions caused by M. pinodes. There was little evidence of cell-wall degradation in limited M. pinodes lesions in which penetration of walls by hyphae seemed to be mechanical in nature. No physical barriers developed in tissues surrounding limited lesions. Nevertheless, A. pisi was rarely found beyond the necrotic area. This suggested that tissues beyond the lesion had become resistant to the parasite. In contrast, M. pinodes often grew outside the necrotic area, sometimes many days after this had stopped growing, but when it did so it caused no more necrosis unless leaflets were placed in conditions in which the spreading type of lesion could develop.

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