Endophytes & weed biocontrol
Californian thistle seedlings, pre-treated with fungal endophytes, are inoculated
with Sclerotinia sclerotiorum, a potential mycoherbicide for controlling this weed
Many plant-pathogenic fungi have potential as bio-herbicides but their efficacy in field trials is often highly variable. We have found that other fungi, that live inside the weed Californian thistle (Cirsium arvense), can either promote or inhibit the activity of a bioherbicide
fungus being considered for use against this weed.
Fungal endophytes are symptomless microbes which live inside all plants. Some of these endophytes have been shown to be beneficial, increasing the host plant’s environmental tolerances or increasing resistance to pests and diseases, including ones used as biocontrol agents.
With these ideas in mind we started investigating the endophytes present
in Californian thistle regarding whether they could impact the effect of biocontrol
agents. Over 60 fungal species were isolated using both culturing and molecular
methods (a technique called DGGE).
The endophytes identified include a range of types from saprobes, which mostly live off dead material, to known pathogens of Californian thistle and other plants, such as grasses. One of the species detected is a rust mycoparasite (Eudarluca caricis). The presence of this sort of endophyte, which is a pathogen of rust fungi, could make it difficult for rust diseases to infect the plant. This is significant because rusts often make good biocontrol agents, and it might explain the localised impact of the Californian thistle rust (Puccinia punctiformis) on this weed.
Daniel Than (Landcare Research Scientist) analyses a DGGE gel
Having identified the endophytes in Californian thistle the next step was to work out whether they influence the activity of biocontrol agents proposed for this weed. Preliminary glasshouse trials were conducted to assess their influence on Sclerotinia sclerotiorum, which causes white soft rot disease in Californian thistle. In the field the fungus has an inconsistent effect on Californian thistle when applied as a mycoherbicide; some plants are killed completely, others recover, and some are not affected at all. Could an endophyte be responsible for this varying result?
Plants were inoculated with one of twenty endophytes. After this the plants were infected with the white soft rot fungus. Some endophytes had no influence on the
disease, some enhanced the disease, and others, such as Colletotrichum acutatum reduced its intensity. Colletotrichum appeared to induce a resistance response in Californian thistle, a bit like immunising it against disease. It is possible that this fungal endophyte, along with others, could be impacting the effectiveness of the Sclerotinia sclerotiorum biocontrol agent.
Those endophytes that promote biocontrol agent activity may potentially be used to improve the reliability of bio-herbicides.