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Causal organism - Oculimacula yallundae and Oculimacula acuformis (formerly Tapesia species)
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The eyespot fungus overwinters mainly on infected stubble. Volunteers and grass weeds can also act as sources of inoculum . P. herpotrichoides can survive on stubble for as long as 3 years which is why a one year break from cereals does not reduce eyespot risk in following crops. Spores of the fungus are often produced throughout the autumn and winter, posing a particular threat to early-sown crops. Infection occurs at temperatures above 5°C and during wet periods. Spores are splashed short distances from infected stubble by rain. Early sown crops are exposed to infection for longer periods allowing more time for the fungus to penetrate the leaf sheaths and cause severe lesions on the stem. The development of symptoms following infection normally takes 6-8 weeks, depending upon environmental conditions. Eyespot can be a serious problem in continuous cereals, where inoculum may build up from year to year, and because a one year break will not reduce inoculum levels, the disease is frequently found in wheat grown after a one year break from cereals.
There are two main types of isolates of P. herpotrichoides, the wheat (W) and rye (R) types. W-types are more pathogenic on wheat and barley than rye and R-types are pathogenic on wheat, barley and rye. In culture, W-types are faster growing and have a smooth margin, R-types tend to be slower growing and have a feathery edge to the colony. R-type eyespot tends to infect later in the season and, because it is slower growing is thought to cause less damage than W-type eyespot.
The sexual stage of the eyespot fungus, Tapesia yallundae, discovered in the early 1990's may play an important part in the life cycle of the fungus. This stage of the fungus is produced on stubble at the end of the season and after harvest. The fungus produces tiny cup shaped structures on stubble which produce air-borne ascospores capable of long distance spread. These spores may allow infection of emerging or young plants which may be in a field thought to be at little or no risk from infection. Set-aside land with cereal stubble left standing may well act as a source of this phase of the fungus and could pose a threat to adjacent cereal crops
Importance
Eyespot is very often underestimated in terms of its importance because
few farmers ever look at the stem bases of crops at the milky ripe stage or
later - when severe eyespot can often be seen. If a crop lodges
then eyespot may be invoked as a possible cause but moderate and severe eyespot
can cause yield loss in the order of 10-30%, even in the absence of lodging.
Where eyespot is severe, lodging can occur - causing problems in harvesting
and frequently a reduction in Hagberg Falling Number.
