Further evidence for host specificity of Colletotrichum truncatum from scentless chamomile

Abstract

Colletotrichum truncatum (CT) is a fungus with the potential for biocontrol of the noxious weed scentless chamomile in western Canada. This fungus is morphologically identical to the anthracnose pathogen of lentil and has occasionally caused slight disease symptoms on lentil in controlled conditions, posing crop safety concern. This study was conducted to better understand the risk and potential infection strategy of the biocontrol agent on the most important pulse crops in western Canada. Inoculation trials were carried out on young plants (3–4 weeks from seeding) and detached leaves from these plants in a greenhouse to determine consistency of cross infection and examine detailed infection processes. The CT isolates from scentless chamomile caused visible disease symptoms only on scentless chamomile, but not on lentil or field pea 14 days post inoculation. Lentil CT isolates caused severe disease on lentil and field pea, but no symptoms on scentless chamomile. Microscopic examinations of inoculated leaves at post-inoculation intervals showed a hemibiotrophic infection process with both chamomile and lentil isolates on their respective hosts, with rapid production of infection vesicles and primary hyphae in epidermal cells that remained alive until the development of secondary hyphae. These infection structures were neither observed in lentil leaves inoculated with chamomile CT isolates, nor vice versa. The initiation of secondary hyphae generally coincided with onset of symptoms on scentless chamomile or lentil plants. Chamomile CT isolates were recovered from 2%–23% of surface-sterilized symptomless lentil leaves incubated on a paraquat agar, and this indicates endophytic colonization or potential latent infection by the chamomile CT in lentil. The possible nature of this quiescent colonization in lentil by the chamomile CT is discussed.