Some dynamics of spread and infection by aeciospores of Puccinia punctiformis,a biological control pathogen of Cirsium arvense

Systemic disease of Cirsium arvense caused by Puccinia punctiformis depends on teliospores, from telia that are formed from uredinia, on C. arvense leaves. Uredinia result from infection of the leaves by aeciospores which are one main source of dispersal of the fungus. However, factors governing aeciospore spread, germination, infection, and conversion to uredinia and telia have not been extensively investigated. In this study, effective spread of aeciospores from a source area in a field was fitted to an exponential decline model with a predicted maximum distance of spread of 30 m from the source area to observed uredinia on one leaf of one C. arvense shoot. However, the greatest number of shoots bearing leaves with uredinia/telia was observed within 12 m of the source area, and there were no such shoots observed beyond 17 m from the source area. Aeciospore germination under laboratory conditions was low, with a maximum of about 10%. Temperatures between 18 °C and 25 °C were most favorable for germination with maximum germination at 22 °C. Temperature and dew point data collected from the Frederick, MD airport indicated that optimum temperatures for aeciospore germination occurred in late spring from about May 18 to June 20. Dew conditions during this period were favorable for aeciospore germination. A total of 122 lower leaves, 2 per shoot, on 61 C. arvense shoots were individually inoculated in a dew tent in a greenhouse by painting suspensions of aeciospores onto the leaves. Of these inoculated leaves, 47 produced uredinia within an average of 21.2 ± 6.9 days after inoculation. Uredinia were also produced, in the absence of dew, on 17 non-inoculated leaves of 12 shoots. These leaves were up to 4 leaves above leaves on the same shoots that had been individually and separately inoculated. Results of PCR tests for the presence of the fungus in non-inoculated leaves that were not bearing uredinia, showed that 44 leaves above inoculated leaves on 27 shoots were positive for the presence of the fungus. These leaves were up to 5 leaves above inoculated leaves on the same shoot. Uredinia production and positive PCR results on leaves above inoculated leaves on the same shoot indicated that aeciospore infection was weakly systemic. In other tests in which all leaves of plants were spray-inoculated with aeciospores, uredinia were produced by 10 days after inoculation and converted to telia and sole production of teliospores in about 63 days after inoculation. Successful systemic aeciospore infections in late spring would be expected to result in uredinia production in excess of a 1:1 ratio of aeciospore infections to uredinia and ultimately telia production in late summer. In this manner, systemic aeciospore infections would promote increased density of telia that lead to systemic infections of roots in the fall.