The principal fungal diseases of pastures can be divided into leaf diseases and root diseases. The major leaf diseases cause leaf spotting, which reduces the area for photosynthesis.
These diseases are caused by a number of pathogens including common species such as Phoma, Leptosphaerulina, Puccinia and Bipolaris spp.
These fungal pathogens are important because they also infect cereal and legume crops and annual pasture species. Perennial pastures act as a source for these fungal pathogens to survive over summer (‘green bridge’).
The risk may be reduced during summer when there is minimal green leaf material present on herbaceous perennial pasture plants.
The biggest problem is when Leptosphaerulina and Phoma spp. are allowed to persist. These pathogens are widespread in WA and can cause major crop losses through defoliation.
The major fungal root diseases found on perennial pasture plants: Rhizoctonia solani, Fusarium spp., Gaeumannomyces graminis var. tritici (take-all) and Sclerotinia sclerotiorum, are common to many crops in WA, although some are host-specific.
These pathogens cause root diseases in most of the perennial pasture species as well as providing a source for diseases to spread to cereal and grain legume crops. The pathogens can survive in soil and crop debris for many years.
The risk is reduced because most perennial pastures, with the exception of lucerne are managed as permanent pastures and are not grown in rotation with annual crops.
Control measures for fungal diseases:
There are four approaches to disease control in annual pastures that are also useful for controlling diseases in perennial pastures:
Cultural control:
Grazing management, rotations, cultivation and sowing healthy seed stocks provide potential control of some leaf disease pathogens. A limitation is that most cultural practices are often determined by production imperatives, rather than disease management.
This is the main method used to control leaf and root diseases in pastures. Grazing management offers the best control options for most fungal leaf diseases while rotation is important for fungal root diseases. Unfortunately, heavy grazing can exacerbate root diseases, as the damage caused by grazing and trampling allows fungal pathogens to enter the crown and cause a rot that leads to plant death.
Chemical control:
Provides good short-term control of some leaf diseases, but provides poor long-term control for herbage diseases. The limitations include pollution, toxic effects on grazing animals, pathogen resistance and the cost.
Host resistance:
The benefit gained from durable, effective, multiple resistance is that there is little further cost once such varieties are established. The limitations from this approach are the lack of availability of such resistance and the time and cost of developing resistant varieties, which could soon become outmoded.
Integrated control:
The benefit from this approach is the complementary effect of methods operating in different ways. Examples include: partial resistance + grazing, or partial resistance + fungicides.
Nematode diseases:
A number of nematodes are pathogenic to perennial pasture plants (Table 3). Most have a wide host range and their population density can increase, especially under perennial grasses. These nematodes can adversely affect cereal yields, e.g. cereal cyst nematode causes heavy losses in cereal crops and is hosted by many grasses.
However, it is easily controlled by crop rotation that ensures the paddock is free of cereals and other grasses. Triticale is an alternative option, as it is resistant to both root lesion nematode and cereal cyst nematode.
The perennial grasses are rarely grown in rotation with crops. Lucerne is often grown in a phase rotation with crops, and nematodes are not a common pathogen of lucerne in WA. Among the greatest risks is the introduction of the stem nematode (Ditylenchus dipsaci) through pasture seed.
Currently, there are no records of stem nematodes having been detected in crops in WA. This nematode would have a devastating impact on crops and could also affect the export of seed as it is seed-borne. Oats and canola are highly susceptible and introduction of this nematode would adversely affect the export hay industry as well as the canola industry.
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