Determining the infection process of Phoma macrostoma that leads to bioherbicidal activity on broadleaved weeds

Isolates of the fungus Phoma macrostoma cause intense photobleaching and mortality of Canada thistle (Cirsium arvense), dandelion (Taraxacum officinale) and other broadleaved weeds when applied to the soil. The symptoms are caused by the production of macrocidins which have been extracted from cultured mycelium and the growth medium broth. The objective of this study was to determine the pathway of infection that leads to plant damage resulting in bioherbicidal activity. This was accomplished by first determining which infective unit (i.e., conidia and/or mycelium) resulted in plant damage when applied to a host. Then the infection process was microscopically observed from infested granules placed in the soil through the various stages of colonization and penetration in a resistant and a susceptible host. Conidia were ineffective as infective units because there was no plant damage when target weeds were inoculated using either a foliar spray of a conidial suspension or granules containing conidia placed in the soil. Only mycelium of the fungus applied either pre-emergently to soil ahead of weed seed emergence or post-emergently to soil containing established weeds resulted in significant plant damage. Microscopic observations showed that P. macrostoma 94-44B mycelium germinated from formulated granules in soil, colonizing roots of dandelion (susceptible) and barley (resistant) within 7 days of application. The fungus entered the hosts at sites proximal to root hairs where wounding of the cells was most likely, and growing intercellularly towards the root core. In dandelion, the mycelium proliferated around the vascular trachea disrupting the competence of neighboring cells. In barley, proliferation was less obvious being restricted to the outer layers and there was no disruption of the internal cell structure. P. macrostoma boasts potential as a bioherbicide to control susceptible broadleaved weeds but not harm resistant nontarget hosts. © 2011.