Competition spotlights spray choices

Press, 9 December 1983, Page 22

Competition spotlights spray choices

By

Dr RON CLOSE

2, reader in plant pathology,

Lincoln College

The cost of sprays used by the nine competitors in

the Wheat 83 competition to control weeds, insects and

diseases varies from $B5 to

$149 per hectare. This represents 28 per cent to 51 per cent of the total costs, or an average of 39 per cent.

Plant protection therefore forms a significant part of the overall costs of production. It is essential to ensure that the correct chemicals are used, that they are applied effectively and at the right time to obtain the best possible control. In solving these problems it is better to prevent them developing than to attempt

to cure them. Thus it is necessary to decide at an early stage the spray programme; which sprays will be needed and when they need to be applied. Such decisions by farmers are based • generally on previous experience with the crop and the pest problem and having some ideas on the level or amount of the pest that necessitates action. Research at Lincoln College and by other groups is trying to define more clearly the times when action is needed, and to develop simple methods of determining the amount of weeds, diseases and insects

in crops. This is especially necessary in relation to knowing when to start spraying for stripe rust.

Fungicides were essential in Wheat 83 to control stripe rust and some competitors were prepared to take the risk of using lower than recommended rates of, Bayleton or Tilt. One of the consequences of using lower rates is that the length of protection is likely to be reduced and spraying is needed more often. For example, five sprays were applied by the Geral-dine-Temuka group to achieve excellent control. In two plots the recommended rate was applied early and then half rates wi*re used later in crop development. It would have been better to have used the lower rate early, and the higher rates when the crops were more fully developed. From the Wheat 83 plots, it would appear that two or three well-timed sprays of recommended rates provided excellent stripe rust control. Timing of the application is vital. In October-Nqyem-

ber the crop is developing rapidly, so that the time between sprays can only be of four to five weeks duration. And it is especially necessary to spray when the flag leaf is fully expanded to protect the upper leaves which contribute most to grain development. Late sprays (after flowering) are not recommended. If it was possible to predict the time of flowering then a spray two weeks before flowering (to protect upper leaves) with a spray four to five weeks earlier could well be the best strategy. At the Wheat 83 field day one competitor stated that sprays of Bayleton and Tilt were used on the same crop in order to prevent stripe rust developing resistance, but this does not happen as both of these are in the same chemical group. To alternate sprays means using a spray from another chemical group and some of these are under test. Rates of Bayleton or Tilt, lower than recommended, are not likely to lead to the development of stripe rust resistance.

In addition, information from overseas indicates that stripe rust has not yet developed resistance even after extensive use of these fungicides. Although Baytan seed treatment was used in all plots, it would seem that in the Lincoln area in 1983 it did not provide the same length of protection of plants against stripe rust as it did in previous years. The length of protection will depend on the season, being influenced by factors such as time of sowing, rainfall, temperature and rate of plant growth. As well large areas of volunteer wheat adjacent to Wheat 83 would have increased the inoculum (spore) pressure on the plots. Seed treatments, to be fully effective, must be applied so that all seeds receive an effective dose. Treatment machinery must be calibrated and used efficiently. Barley yellow dwarf virus was at a very low level in all Wheat 83 plots. Nevertheless three farmers applied insurance sprays of insecticides in August to prevent any possible increase in cereal aphids and in spread of B.Y.D.V. In November, the rosegrain aphid was found in

plots. But in no case had it reached the level on wheat plants where spraying is regarded as necessary, i.e. 50-60 aphids per tiller. An interesting plot was where Oroua was the cultivar grown. In this stripe rust was severe. This contrasts with Wheat 82 when Oroua was not severely affected. A new race of stripe rust, able to infect Oroua is now firmly established in Canterbury, so that this cultivar will need the same protective sprays as RongoThe herbicide used by six out of the nine competitors was Glean. It is an effective herbicide but probably no more so than many other herbicides marketed either as single chemicals or as mixtures. The basic question always is “Was a herbicide spray really necessary?” If costs are the criterion of the competition then perhaps in this field a herbicide could have been omitted from the programme. With such a large proportion of total costs being chemical sprays and their application, such inputs could have a major bearing on the final outcome of the Wheat 83 competition where inputs are balanced against yield to determine the winning plot.