WHEAT DISEASE PROBLEM EXAMINED

Press, Volume CII, Issue 30064, 23 February 1963, Page 6

WHEAT DISEASE PROBLEM EXAMINED

[By Dr. B. C. SMITH, officer in charge. Plant Diseases Division, D-SJ-R., Lincoln}

During the last two months an intensive study has been made ot diseases in wheat crops from Hawke’s Bay to Southland. M has become apparent that over the whole of the whealgrowing area this has been the worst season for diseases for at least 25 years. lit is the first season in which several diseases have been severe in the same district and often on the same crops, and it is very unlikely that such a severe season will occur again for many years. Many farmers have expressed the opinion that our most popular variety, Aotea, has “run out" or degenerated. The same thing was said •of Cross 7 but all available evidence from variety trials throughout New Zeeland suggests thast these varieties do not and have not changed, but it is the diseases that change. For example, when Aotea was first released it showed marked tolerance to many leaf diseases but now that it is grown on more than 30 per cent, of the wheat area, jit has become severely attacked by several of these leaf diseases. Therefore, until we have perfected new methods of testing for leaf disease resistance it appears that new varieties will continue to suffer from “poor seasons” from time to time.

Perhaps one of the best ways of preventing this kind of disease situation from arising is for farmers to grow at least two different varieties of wheat wherever possible. The different wheat diseases varied in importance in various districts. In the Hawke’s Bay and Wairarapa areas the order of relative importance was yellow dwarf virus, Argentine stem weevil, leaf rust and- mildew, with the average loss in yield about 40 per cent In the Manawatu, yellow dwarf virus was most important followed by stem rust Argentine stem weevil, and leaf

rust, with an average loss in yield of about 40 per cent. Many crops of Gabo suffered from severe stem rust attack reducing yields from 60 to 100 per cent In Marlborough the disease situation was similar to Manawatu, except that stem rust was most -important (giving 00 to 100 per cent, loss in yield in most Hilgendorf (1961) crops) followed by yellow dwarf virus, and leaf rust, with an average loss in yield of about 45 per cent.

North Canterbury suffered severely from yellow dwarf virus in all varieties, stem rust was severe in Hilgendorf (1961) and Arawa, and leaf rust and mildew severe in Aotea, with an average loss in yield also of about 40 per cent.

Mid-Canterbury suffered moderate losses from yellow dwarf virus, mildew, leaf rust, take-all and Septaria in that order, • with an average loss in yield over all crops of about 30 per cent. South Canterbury and Otago suffered moat from yellow dwarf virus on all varieties, from mildew and leaf rust on Aotea, and from stem rust on Hilgendorf (1961) in some inland areas. Southland had the most healthy crops of all districts in New Zealand and this should be reflected in. high average yields for that province. In Southland mildew and leaf rust were the most important diseases. Yellow dwarf virus was found in a moderate to severe degree only on a few autumn-sown wheat crops. Unlike the North Island there appears to be no large spring flight of cereal aphides to cause severe yellow dwarf virus infection' in spring-sown cereal crops in Southland, and this, I think, is largely responsible for the consistently high yields obtained in Southland with spring-sown cereal crops. To avoid disease losses

next year ft would appear that the best wheat varieties to grow in the different districts would be as follows: East coast of North Island —Sowing before midSeptember. Aotea, Hilgendorf (1961), Cross 7 (1961). After mid-September; Mengavi or Gabo. West Coast of North Island—Slowing before midSeptember: Aotea. After mid-September: Mengavi.

Marlborough—Aotea. North Canterbury Aotea and Hilgendorf (1991). Mid-Canterbury Hilgendorf (1961), Arawa, Aotea or Cross 7 (1961).

Otago—Coastal: Hilgendorf (1961), Aotea. Inland: Aotea. Southland Hilgendorf (1961), Cross 7 (1961), or Aotea. The only recommended disease contact measures are: (1) All seed to be treated with a mercury or combination mercury type seed dressing, to prevent the spread of stinking smut, a disease common in early wheat growing in New Zealand which had disappeared for more than 12 years but has shown up again in Central Otago this season.

(2) AU wheat crops to be inspected for aphides four to six weeks after sowing and sprayed if aphides are present to control yellow dwarf virus, which is the most important disease affecting wheat in New Zealand.

The survey of farmers’ wheat yields in North Canterbury to date has given further confirmation of the loss in yield caused by aphides spreading virus and the value of spraying. The average yield of 27 unsprayed crops which had no aphides was 37.8 bushels and of nine unsprayed crops with slight aphis was 27.3 bushels. Where crops had been sprayed the yield over 17 crops with slight aphis infection was 36 bushels and in 20 crops with heavy infection 31 bushels to the acre.