Eyespot

Pathogen

  • Oculimacula yallundae (Helgardia herpotrichoides)
  • O. acuformis (H.acuformis)

Hosts

The disease affects wheat, barley, oats, rye and triticale.

  • O. yallundae (W-type) is more pathogenic on wheat and barley than on rye.
  • O. acuformis (R-type) is path-ogenic on wheat, barley, rye and triticale.

Symptoms

Early symptoms can be confused with sharp eyespot and Fusarium spp. Frequently all that is visible is a brown smudge on the leaf sheath at the stem-base. If crops are early-sown eyespot lesions may penetrate one or two leaf sheaths, making identification more conclusive. Lesions caused by fusarium spp. and sharp eyespot are frequently confined to the outer leaf sheath. Later in the season eyespot symptoms become more distinct and appear as an eye-shaped lesion with a dark margin, usually below the first node. Later still, the margin of the eyespot lesion is often dark and diffuse with a central black 'pupil' occasionally visible. In severe attacks of eyespot, white-heads ("bleached" ears) are commonly seen scattered through the crop, later in the season these may become colonized by sooty moulds.

Eyespot tends to be more severe if plants are also suffering from take-all.

Life Cycle

Eyespot-life-cycle.gif

The fungus overwinters on infected stubble, volunteers and grass weeds can also act as sources of inoculum. It can survive on stubble for as long as 3 years which is why a one year break from cereals does not necessarily reduce eyespot risk in following crops. Spores are produced throughout the autumn and winter, posing a threat to early-sown crops. Infection occurs at temperatures above 5OC and during wet periods. Spores are rain splashed short distances from infected stubble. 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.

The sexual stage of both eyespot fungi, play an important part in the pathogen life cycle. This stage of the fungus is produced on stubble at the end of the season and after harvest, ascospores may travel long distances and infect emerging or young plants.

Importance

Eyespot is very often underestimated in 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. Moderate or severe eyespot infections 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.

Cultural control

A two year break from cereals will frequently reduce inoculum in a field to a level which does not pose a serious threat to the following crop. A one year break from cereals is not sufficient to significantly reduce inoculum levels. Minimal cultivation following a cereal crop, as an introduction for oilseed rape can be used to reduce eyespot inoculum. Cereal stubble is left on the soil surface during the oilseed rape crop and is then ploughed down in preparation for another cereal crop. The infective stubble, having produced spores throughout the oilseed rape crop, is likely to pose less of a threat than inoculum on fresh stubble. Ploughing down in preparation for the next cereal crop after the oilseed rape, further decreases the inoculum risk. Ploughing releases nitrogen and can increase risk from eyespot. Minimum tillage and direct drilling can pose less of a risk from eyespot.

In the UK the resistance of winter wheat to eyespot has been derived mainly from the variety Cappelle Desprez. This confers a degree of tolerance rather than total resistance to disease. The introduction of the winter wheat variety Rendezvous in the 1980's brought a new source of resistance derived from a grass species, Aegilops ventricosa, which is now used in many breeding programmes. Varieties carrying high levels of resistance (Pch 1 gene) are now available.

Photo gallery

To view the gallery please click here