Eyespot disease of cereal

Eyespot (Pseudocercosporella herpotrichoides ), a serious disease of winter wheat and barley, has increased in importance in the 1980s because I) pathogen populations have become resistant to the fungicides which were used to control it; 2) changes in cropping practices, such as earlier sowing dates, have favoured the development of severe epidemics. In 1987, ADAS national surveys suggested that, of the diseases surveyed, eyespot was the most serious disease problem on winter wheat, with losses estimated at c.£30M despite the widespread application of fungicides against eyespot (cost c. £23M). These figures are underestimates since they include neither the losses caused by eyespot-induced lodging, nor the costs of fungicide application. Furthermore, it is likely that eyespot has been an important factor in reducing the quality of milling wheat and malting barley crops so that they have to be sold for animal feed. Since the formula used by ADAS to estimate losses for winter wheat was derived both varieties and eyespot populations have changed, so experiments are needed to re-evaluate it. A national estimate of losses from eyespot-induced lodging is needed and a formula needs to be derived for winter barley.

The disease takes its name from the characteristic oval brown-bordered lesions which form at the base of the shoots. When severe, lesions may weaken the stem bases so that a crop may lodge. Eyespot attacks all commercially grown varieties of wheat and barley although they vary greatly in resistance. Rye and triticale are susceptible to some strains of the fungus; oats and grasses may be infected but lesions are not damaging. It is a disease of temperate cereal crops, and occurs throughout Europe, in North America, Australasia and South Africa. In the UK it is particularly damaging on autumn-sown crops. Stem bases of cereal crops may also be affected by sharp eyespot (Rhizoctonia cerealis) or brown footrot (Fusarium spp.) but these diseases are considered to be less serious than eyespot, although it can sometimes be difficult to distinguish the different cereal stem base pathogens present on plants sampled from crops.

The eyespot fungus, P. herpotrichoides, has two main pathotypes. W-type (wheat-type) isolates are generally more damaging to wheat than to barley and much less damaging to rye. They produce colonies with smooth edges when grown in culture on potato dextrose agar. R-type (rye-type) isolates are about equally damaging to wheat, barley and rye. They produce colonies with feathery, edges and grow more slowly than W-types. However, it is not clear how these pathotypes relate to two varieties of P. herpotrichoides described in West Germany and distinguished on the basis of spore characteristics. Furthermore, another related species, Pseudocercosporella anguioides, can be isolated from leaf sheath lesions but does not colonize the stem and appears not to be damaging. However, it may confuse identification of eyespot lesions, and result in unnecessary spray applications. Since the eyespot fungus has now developed resistance to the MBC fungicides which were used to control it, isolates from lesions on crops may be MBC-sensitive or MBC-resistant. It is important to distinguish quickly between eyespot and other stem bases pathogens and between the different strains of the eyespot fungus since spray decisions may be determined by the identity of the fungus present on stem bases of a crop. There is an urgent need for rapid diagnostic methods for use on the stem base pathogens to be developed.

There has been a dramatic change in populations of the eyespot fungus in the UK over the last decade. Formerly MBC-sensitive W-types were predominant but now MBC-resistant R-types are predominant. The development of MBC-resistance has occurred because MBC fungicides were used routinely by growers but is less clear why the predominant pathotype should have changed. However, surveys suggest that the composition of eyespot populations in other EEC countries is different and that different population changes have occurred (e.g. MBC-resistant W-types are predominant in France). A series of comparative surveys in different countries might indicate why these differences between populations in different countries have occurred and suggest methods of manipulating eyespot populations for the benefit of European agriculture.

Epidemics of eyespot can be considered in five phases: survival on infected straw from previous crops, sporulation, dispersal by rain-splash, infection of plants in new crops and lesion development.

In the UK conditions are often favourable for sporulation dispersal and infection during the winter and where inoculum is available, lesion development is often the limiting phase which determines whether or not epidemics will become severe. The crucial stage in lesion development is the initial colonization of the stem from the infected leaf sheaths which surround it. If the fungus becomes established in the stem further lesion development may be a function of temperature but if infected leaf sheaths die before the stem has been colonized lesions may fail to develop further. It is important to forecast the development of severe epidemics so that sprays are only applied when they are economically justified. Yield losses occur only when there is a high incidence of moderate or severe lesions during grain-filling but sprays need to be applied much earlier than this to control the disease. Current methods for forecasting the severity of eyespot epidemics at a time when sprays can be applied have not always been reliable. In some years when sprays have been recommended, eyespot has not become damaging and in others when sprays have not been recommended severe late epidemics have developed. Current work, funded by H-GCA, is aiming to integrate methods for forecasting eyespot and septoria diseases of winter wheat but will not be able to answer all the important questions in the time available. There is, for example, an urgent need to determine more accurately the threshold criteria to be used in deciding whether or not to apply a fungicide spray.

The priorities for future research on eyespot are:

1. To improve estimates of national losses from eyespot.

2. To improve the accuracy of methods for predicting the severity of eyespot epidemics.

3. To compare populations of the eyespot fungus in different EEC countries.

4. To study the comparative biology of the pathotypes of Pseudocercosporella herpotrichoides, the eyespot fungus, and of P. anguioides.

5. To develop rapid diagnostic methods for distinguishing 1) between the eyespot fungus and other stem base pathogens; 2) between the pathotypes of P. herpotrichoides.

6. To develop new varieties with different sources of resistance against eyespot and new fungicides for control of eyespot, and to evaluate methods for improving the efficiency of existing fungicides.

In order to investigate these problems most effectively close collaboration between scientists in ADAS and AFRC will be needed for much of this proposed research. Furthermore, collaborative ADAS/AFRC research will also be needed on other stem base diseases of cereals. To deploy resources most effectively it seems advisable to designate one centre for co-ordination of research on stem base diseases of cereals within ADAS and AFRC.

This review, completed in March 1988 has 88 pages in the full article.