MAF PLANT HEALTH MONITOR

Press, 30 August 1985, Page 10

MAF PLANT HEALTH MONITOR

Some confusion was caused by the last Plant Health Monitor concerning the treatment of scald and net-blotch in autumn sown barley. Scald, caused by Rhynchosporium secalis, is a problem in cool, damp, windy conditions, and is now showing up on many barley crops. Winter barley cultivars are bred for resistance to this disease (e.g. Illia and Priver), but if spring barley cultivars such as Triumph are sown early, they can experience severe problems. Seed treatments can provide partial control. Baytan IM controls any seed-borne infection, and can give early protection of emerging foliage because of its. systemic action.

The disease can be controlled by Tilt or Bayleton once infection has been found in a crop, and when there is still a chance of any persistent cold wet weather.

Net-blotch, caused by Drechslera teres, is showing up on first leaves of many crops. Normally this disease is controlled by seed treatment, (Baytan IM, Fungazil, Vitaflo or Dithane M 45) but it can occur even after seed treatment.

Growers need not apply a foliar fungicide until they observe the disease spreading from the first leaf onto new leaves since it often never progresses past this stage in the South Island. Tilt is the only foliar fungicide with net-blotch registration at the moment.

If net-blotch is to be controlled, do not use Bayleton after Baytan seed treatment, and will ensure that resistant strains of the fungus are not encouraged. Many barley foliage diseases such as scald, rust, and net-blotch, can spread from crop debris or disease ridden volunteers that overwinter these problems. Every effort should be made to reduce the “greenbridge” that allow the

diseases to carry-over. Get rid of barley volunteers by grazing down old pasture, spraying fence lines, or ploughing. CEREALS ® Stripe rust is showing up in some early sown crops and is likely to now be in the latent stage in most autumn sown crops. Monitor crops carefully from now on, especially susceptible cultivars, and apply a fungicide at the first sign of disease. Note that Oroua, Rongotea Weka, and Advantage are all susceptible in the seedling stage but Weka and Advantage have adult resistence. • Adult Argentine stem weevils are now active and will cause minor damage to leaf tips. Look for small (approx 2mm) windows in chewed leaf blades and black elongated eggs may also be seen on tillers at the base of leaves. Adult stem weevil damage is not significant in cereals and no action is required. • Beware of bird damage in emerging crops. Plants may be cut or pulled out of the ground. If the problem is considered serious, determine the extent of damage and report it to your local M.A.F. advisory officer. • Where grass grub larvae have damaged crops in distinct patches, apply nitrogen to the damaged areas to help plants reestablish, and compensate for pest damage. PEAS • Testing of soil samples for root-rot (aphanomyces) potential has now been completed at the Lincoln Plant Protection Centre. A total of 405 samples were submitted, 92 per cent from MidCanterbury. Sixty-seven per cent of the samples showed a low damage potential index, 10 per cent showed possible damage potential, while 23 per cent were rated as high risk. Growers are advised to consult results of the most recent Aphanomyces-poten-tial tests for each paddock

before planting peas. Under warm, wet conditions in late spring, this disease can cause significant losses. BRASSICA SEED CROPS © Check for aphids, and if they are found easily, control before flowering starts. Remember that any insecticide applied during flowering will kill bees. © Check crops now for the presence of leaf spots, as these diseases may later infect seed pods reducing quality and yield. Bravo could be applied with insecticides before flowering to control Alternaria and Ring-spot. PASTURE 9 Consider the use of nitrogen on grassgrub damaged pasture. The extra nutrient will stimulate more growth oh undamaged areas and help promote root growth in damaged patches. Direct drill ryegrass into pest damaged pasture now. In most cases an ordinary drill will do the job, particularly if the insects have bared the soil surface. Use 10kg to 15kg per hectare of grass seed which has been treated with a mixture of Captan (3g a.i.per Ikg of seed). A light application of either glyphosate or paraquat before drilling is suggested to reduce resident plant competition. Contact your M.A.F. office for details.