Effects of Leaf Maturity, Infection Site, and Application Rate of Alternaria cirsinoxia Conidia on Infection of Canada Thistle (Cirsium arvense)

Abstract

The effects of leaf maturity, infection site, and application rate of Alternaria cirsinoxia conidia on the pre- and postpenetration phases of infection of Canada thistle were studied. Leaf maturity had no effect on the germination of conidia, but appressoria formation was significantly higher on the oldest leaf than on the youngest leaf. There were no differences in the frequency of leaf penetration in the youngest and the oldest leaves, but consistently larger lesions formed on the oldest leaf at 10 days after inoculation than on the youngest leaf. Penetration of the oldest leaf was greatest at the leaf tip, compared with the midleaf area or leaf base. Increasing the application rate of A. cirsinoxia conidia (105/ml) from 200 to 800 L/ha, with either continuous leaf wetness or intermittent drying during application, resulted in increasing conidial densities on the phylloplane from 3–4 to 9–13 conidia per 1.67-mm2 field of view. Intermittent drying during application reduced germination of conidia, but leaf penetration did not differ greatly among treatments. Disease severity on whole plants at 14 days after inoculation was not affected by application rate or leaf-wetness treatment. All plants scored 5–6 on the Horsfall–Barratt rating scale, with severe infection occurring only on the older, basal leaves. Thus, the plants recovered by developing new leaves at the apical portion. These results show that A. cirsinoxia is primarily pathogenic on older, senescing leaves of Canada thistle, irrespective of the application rates tested on whole plants. This characteristic limits its potential as a bioherbicide for Canada thistle.