Survival and dissemination of conidia of Discula destructiva Redlin through the alimentary canal of an insect

Discula destructiva Redlin, the cause of dogwood anthracnose on Cornus florida L., is known to be spread by wind and rain. The rapid spread of this pathogen suggests that insects also may be important in the dissemination of D. destructiva. Viable conidia of D. destructiva can be carried externally and internally by convergent lady beetles (CLBs), Hippodamia convergens Guerin-Meneville. It is not known if conidia carried internally are discharged in viable condition, and if so, are capable of infecting susceptible hosts. Using CLBs as a model insect, the objectives of this research were to: 1) develop a nonlethal method of surface-disinfecting adult CLBs, 2) determine if the beetles can discharge viable conidia of D. destructiva in frass, and if so, for how long, 3) quantify the amount of conidia in frass, and 4) determine if the beetles, exposed to conidia of D. destructive and then surface-disinfested, can initiate infection of healthy dogwood leaves in the greenhouse, by discharging viable conidia of D. destructiva in frass. Beetles were surface-disinfested after exposure to D. destructiva to ensure that their frass would not be contaminated by conidia carried externally. Seven different durations of disinfestation were tested (0 s, 15 s, 30 s, 45 s, 60 s, 2 min, and 5min), using a solution of 0.525% sodium hypochlorite and 5% ethanol. Disinfestation of the beetles was always followed by a 15 s rinse in sterile water. Ten percent of the water used to rinse the beetles was tested for viable conidia. The 5 min disinfestation was used as the standard disinfestation period for all subsequent experiments because it was the longest disinfestation period that results in ≤ 50% beetle mortality and the absence of viable propagules of D. destructiva in the beetle rinse water. Mortality caused by the 5 min disinfestation averaged 24% for all subsequent experiments. To determine if conidia of D. destructive were discharged in frass in viable condition, beetles were exposed to sporulating cultures of D. destructiva for 1 h and then surface-disinfested. Frass was collected after disinfestation and tested for the presence of viable conidia. Seventy-six percent of the beetles tested discharged viable conidia of D. destructive in their frass. More than 80%, 32%, 19%, 13%, and 12% of the beetles from which D. destructive was isolated retained viable conidia of D. destructiva internally for at least 12, 24, 48, 72, and 96 h, respectively. A hermacytometer slide was used to quantify conidia of D. destructiva in frass. The numbers of conidia varied greatly, randing from 0 to 3.2 x 106 conidia/frass pellet. In a greenhouse inoculation chamber, conidia of D. destructiva discharged in frass of CLBs were capable of infecting susceptible dogwood leaf surfaces. Sixty-eight percent of branches treated with beetles exposed to D. destructiva and 32% of branches treated with beetles exposed to D. destructiva and then surface-disinfested became infected with D. destructiva. No infection by D. destructiva occurred on branches with non-infested beetles and no beetles (controls). Insects may contribute to the spread of dogwood anthracnose. A variety of insects can acquire and transport viable conidia of D. destructiva externally in the field. These results suggest that insects also may pass viable conidia of D. destructiva in their frass and thereby inoculate leaves of flowering dogwood resulting in symptoms of dogwood anthracnose