Farm and Station. Protecting the wheat crop against disease

Press, 9 May 1980, Page 8

Farm and Station. Protecting the wheat crop against disease

By this time of the year most farmers will have decided on the area of autumn-sown wheat they are going to grow, and where it is to be sown. As a result of the increased price per tonne,' the over-all area is likelv to be increased.

Farmers also should be deciding on an over-all plant protection programme. The earlier this is done (preferably before sowing) the more control strategies are available. Such strategies cover what to grow, how to grow it, and how to look after it. The variety grown is chosen more in relation to the potential or actual market outlet for the grain, taking both yield and. quality into account. However, it is worth thinking about, varieties that resist disease, for the cheapest and best means of disease control is the use of such resistant varieties. For example:

Takahe and Arawa are mildew resistant.

Rongotea and Oroua are leaf-rust resistant, and have some resistance to speckled leaf blotch. Aotea and Takahe have some resistance to eyespot (use these .as second crops).

These varieties should be chosen if the diseases mentioned have been a cause of serious yield losses in the past.

Grain for seed can be saved from your own crop or can be obtained from merchants as . certified seed (of which there are three grades). All seed wheat should be treated with a fungicide. Seed from merchants will be treated. Farm-saved seed may be sent to merchants for treatment, or be treated by mobile operators, or by the farmer. It is known that some farmers have used untreated seed, which has come from a crop grown the previous year from certified seed. While farmers may get away with this, it is not a good practice and certainly should not be done more than once with any line of seed. It is also necessary for the smaller grains in the line to be dressed out as these can produce weak seedlings more prone to disease.

Five seed treatment chemicals are available. Orthocide 75 and Dithane M 45 will control stinking smut (bunt) and protect the plants against some soil-borne disease organisms. Granosan, Vitaflo 200 and Baytan Fl 7 will do the same but also control loose smut, an infection that can be present in the seed. Baytan Fl 7 also protects the young crop from powdery mildew and leaf rust, diseases that are more common in the young, spring-sown wheat crop. Many disease organisms can survive in the previous crop residue, particularly take-all, eyespot, mildew, and speckled leaf blotch. Hence the strong recommendation to burn the stubble, and to avoid growing wheat after wheat wherever possible. Even where a good burn is obtained. disease inoculum may persist in the headlands where diseased

straw has been ploughed in to form a firebreak.

It is possible to grow two wheat crops in succession or even three, but the increase in amount of take-all, eyespot and such insects as Hessian fly will cause real problems and yield loss in these and subsequent crops.

The break between wheat crops in a rotation should be at least two years without wheat or barley, because take-all and eyespot also survive on barley residues, though barley is less severely affected by these diseases. Many farms are becoming more and more crop-ori-ented, so such a break is difficult to achieve. Take-all can also survive on couch (or twitch). Areas with severe couch problems should be avoided for wheat cropping as couch is often infected with take-all. Killing the couch by spraying just before sowing does not solve the problem. You have to wait until the couch roots have rotted to be rid of take-all. Note that if the area chosen for wheat is out of pasture, there could be grass grubs and other soil insects present which will damage the roots of the young crop. Thus grass grub control treatments may be essential prior to or at drilling. The recommendation to sow at the end of May or early in June for virus control still applies.- This ensures that the crop emerges after the cereal aphids have stopped flying, and so autumn infection with cereal yellow dwarf virus is avoided. Because of difficult soil conditions and other reasons some farmers have to sow early in May. In these cases a granular insecticide for aphid control should be applied at sowing or the crop should be sprayed in mid-August to control aphids to stop the spread of virus. This insecticide spray could be combined with that for weed control.

Self-sown cereal plants may grow from seed that has shaken from the ears or fallen from the header and then been incorporated into the soil during the cultivation for early autumn-sown pastures or crops. These plants can be an important source of mildew and leaf rust spores, as well as of aphids — both cereal and grain aphid. It is essential for these to be eliminated as much as possible by hard grazing. Barley volunteer plants should also be grazed, though the mildew and leaf rust on barley will not attack wheat, and those on wheat will not attack barley.

Let us assume that as a farmer you have followed all these rules. You have bought good treated seed, sown it at the correct depth (5 cm), at the right time and in the right

amount, and in an area not previously in wheat. Should you now be able to forget about your crop until harvest time? No. With the emphasis on the use of nitrogen and irrigation for maximum yields, the crop does need to be examined from time to time. The first thing is an emergence count to see that you have the right number of plants per square metre (250-300), likely to give the optimum number of heads or ears at harvest. Some thoughts should be given to weed control early in the season, especially if you want to change your control programme and use different chemicals. Check on the properties of the large range of chemicals or combinations now available, try’ and decide what is best for you, and if necessary purchase stocks in advance. Farmers may need to wait and see what weeds are going to be present, but from past experience most will be aware of their problem weeds. Weeds also increase the humidity within crops, and this can increase the amount of some diseases.

Vigilance against diseases is important. During the period of tillering (growth stages 3-5) infections of speckled leaf blotch can occur and cause considerable damage to leaves. When infected, they tum yellow, with brown areas present in which are embedded small black spore-producing bodies known as pycnidia. It is a disease readily identified in the field. In 1979, research by Dr Sanderson, of D.5.1.R., and Dr Gaunt, of Lincoln College, showed that this disease was controlled satisfactorily by sprays of Benlate plus I per cent Universal Spraying Oil, by Bayleton and to some extent by Bravo. It is suggested that these chemicals could be applied at the same time as the weedkiller sprays. Care

should be taken to ensure that the various chemicals used are compatible (can be mixed together), that the mixture does not damage plants, and that effective control of weeds and disease is still being obtained.

Last year speckled leaf blotch was severe in early spring (late August — early September), but from past experience it is known that the severity of this early attack varies from one season to another.

Thus the decision must be either to use spray treatments in early . spring as an insurance or to treat part of your crop to assess for yourself the damage that may be caused by this pathogen. It is early days in research on this problem and until there are good methods of predicting disease outbreaks and losses, farmers should be cautious in adopting over-all spray programmes. They should try to evaluate spray treatments on their own farm; spray half paddock areas and, at harvest, try and get some idea of the differences in grain yield,

grain size, and quality from comparable harvest blocks.

In November, speckled leaf blotch can spread onto the top leaves • (by rain splash). If the disease appears on the flag and second leaf then a Bayleton spray could be applied, especially as it also controls mildew and leaf rust. Powdery’ mildew is another disease that can be found in wheat during the late spring. The conclusion from a' large number of trials (by D.S.I.R. and others) is that in general it is not worth treating wheat for the control of this disease. However, there could be some crops, especially of Hilgendorf and Aotea, which would be well worth spraying; but it is always difficult to make this decision.

Ini growing wheat, there could be advantages from spraying on a growth regulator. The only material available is chlormequat, marketed as Cycocel, a product used extensively in Europe. It is suitable for use on wheat and oats and is best applied during G.S. 6 to 7, the stem extension phase when the first and second nodes can be felt at the base of the stem. Cycocel helps to limit straw growth, and so makes the crop easier to harvest. It also thickens the stem so that lodging and stem break is less likely to occur. This is an advantage when the use of nitrogen has stimulated growth or a light infection of eyespot is expected. Where severe infections of eyespot are expected, then Benlate sprays will give good control. A further problem is leaf rust, which may attack the wheat crop during the later stages of plant development, from the boot stage (G.S. 9) onwards. The fungicide Bayleton is. excellent for rust control. The problem is knowing whether to spray or not. Obviously the disease must be present before spraying is worthwhile, so the crop must be inspected. To avoid routine spraying of all crops, some rule-of-thumb is needed to determine when to spray. Good research experience is not available with leaf rust of wheat, but some guidance can be

obtained from work on leaf rust of barley. A representative and unbiased sample (30 to 50 or more tillers) is obtained from the crop at a time when all the ears have emerged and prior to flowering (G.S. 10.5). The flag and second leaf are examined for leaf rust. If more than half of these leaves are found to be infected (two or more pustules), then spraying would be warranted. If this level of infection is not reached till much later (flowering complete and grain milky ripe, G.S. 11.1) then spraying is going to be of less value. It is hoped that further research will clarify the recommendations relating to leaf rust control in wheat. Stem rust is a disease which is not generally a problem. In some years and in some localities it may cause severe losses. Again accurate diagnosis is important, and if it is found infecting all stems at growth stage 10.5, then spray with Bayleton. Similarly grain aphids sometimes can be a problem as they infest the developing ear and feed on the young grain. At one time these aphids caused considerable damage. Isolated outbreaks still can occur; spray if 30 per cent of the heads are found to be infested. The main message is that farmers should inspect their crops regularly and get as much advice as possible. If you do decide to spray a crop, then knowing the costs involved, you should try and get some idea of the benefits obtained. In tins way you will gain valuable experience to use in later years.