The wheat book : principles and practice - Department of Agriculture ...

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CHAPTER 10 – DISEASES THE WHEAT BOOK Root disease ..................................................................................................217 Principles and management ................................................................................217 Introduction ....................................................................................................217 Rotation ..........................................................................................................217 Tillage ..............................................................................................................217 Seasonal influences ..........................................................................................218 Soil type ..........................................................................................................219 Nutrition ........................................................................................................219 Host resistance ................................................................................................219 Chemical control ............................................................................................219 Take-all ............................................................................................................220 Rhizoctonia bare patch ....................................................................................222 Fusarium crown rot ........................................................................................223 Root lesion nematode ......................................................................................224 Cereal cyst nematode ......................................................................................226 Common root rot ............................................................................................227 Pythium root rot ..............................................................................................228 202
THE WHEAT BOOK CHAPTER 10 – DISEASES LEAF DISEASE Principles of management Leaf diseases are a universal phenomenon in Western Australian wheat crops. Their occurrence, severity and impact vary across regions in response to management and seasonal conditions. Rapid disease build-up and even spread within crops are general features of the biology and life cycle of most leaf diseases which undergo several infection cycles in a season and are readily dispersed in air or wind blown rain splash. Management requirements vary with disease risk and potential for damage. Cropping practices are under continual development and several of them, such as early sowing, optimal nutritional management and increased stubble retention, can result in increased disease pressures. Most wheat leaf diseases only attack wheat. This narrow host range assists in disease management options but the capacity of many leaf diseases to develop quickly under favourable conditions means that combining strategies that reduce risk of losses is usually required for optimal management. Integrating leaf disease management options is the key to successfully minimising effects of leaf diseases of cereals. Rotation Common leaf diseases such as septoria blotches and yellow spot survive and spread from infected stubble residues. One of the many advantages of crop rotation is that infected stubble residues are separated from the current crop, thus reducing disease risk. Separation of crop from disease source occurs both in space and time. Without rotation a second wheat crop is placed in close proximity to the diseased stubble from the previous crop. This guarantees that disease spread into the current crop is early and possibly severe unless stubble is removed (thoroughly burnt or ploughed in). For this reason crop rotation is an important facet of stubble retention systems. In a 1:1 rotation, the next wheat phase of the rotation is established 18 months or more after the harvest of the previous wheat crop. This time period is important in allowing the breakdown of the infected stubble residues. Even in 1:1 wheat-lupin rotations where stubble is retained only about 5% of wheat residue remains after the sowing of the return wheat crop. This remaining residue is less infective than fresh residues that have survived for just one summer. The overall effect of reduced quantity and infectivity of residues after rotation is that they induce little, if any, disease in most situations. How disease originates The fungi that cause leaf spot diseases such as septoria and yellow spot survive in stubble and become active in the autumn, producing spores as the season opens. Most of these spores are dispersed short distances so doublecropped wheat can be infected severely. 203 Rotated crops avoid severe early disease but still become infected because other spores, dispersed long distances, are able to infect crops far away. This type of infection is usually mild initially, but provides an infective base from which the disease can increase as the season progresses if conditions are suitable. Rust fungi require green living hosts to survive. Spores of wheat rust diseases produced on oversummering host plants (mostly wheat) are also spread by wind to new wheat crops. The earlier disease originates in a crop and the intensity of this early infection are important determinants of how severe the disease becomes as the crop grows. Initial effects of early sowing on disease While all wheat crops are subject to airborne spores from remote sources, early sown crops are exposed to infection before later sown crops, giving disease a head start. In April and early May sown crops the initial infections develop in mild autumn conditions allowing the fungus to mature quicker than in later sown crops whose initial infections develop in the colder winter months. Early sown crops have greater initial disease because they have been exposed to more infections than later sown crops and these infections have been able to advance further before the winter sets in. Moderate to severe leaf disease in young wheat crops can reduce early growth, but leaf disease of wheat in a rotation, which included legumes or oilseeds, would not generally be severe enough to significantly affect early growth. The importance of disease in the early stage is as a source of infection later in the season. Crop development and re-infection Until late stages of stem elongation each stem produces new leaves continuously. Although disease spreads onto the new leaves, low levels of disease do not have a significant direct effect on yield. Disease progresses up the plant gradually as spores produced on lower leaves spread the fungi to newer leaves. Plants may appear to outgrow the disease after winter as stems elongate and leaves develop rapidly. At this time the rate of development of new tissue exceeds the rate of death from new infections. However, this is a transitory phase. Infection will continue to develop after the plant has stopped producing new leaves and the disease will be able to ‘catch up’. It is at this stage that most of the differences in disease development between early and later sown crops occur. The top two or three leaves on the wheat stem are crucial for grain production. Once produced they must remain green for as long as possible to ensure the plant completes grain filling. From the moment the upper leaves are unfurled they are exposed to infection. The third top leaf (the second
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