Plant health monitor

Press, 3 December 1982, Page 18

Plant health monitor

The Ag Phone service has now been operating for two weeks. In future, topics will be listed in the Plant Health

Monitor. However, updating could occur more often. Each week growers are invited to ring the AgPhone number 66887 to check currently available information.

Topics for next week are: • Control of clover insect pests. • Mite control in apples. •-Porina and grass-grub control by management (repeat). • Update on control of rose-grain aphid.

• Control of the common grain aphid in wheat. • Testing- for Argentine stem weevil damage to pasture.

• Bacterial problems in glasshouse tomatoes. The recent' period of dry hot windy nor’west conditions has caused leaf tip death’ in grasses and cereals. If these symptoms are encountered don’t necessarily blame a herbicide or a new disease. WHEAT

• If a second spray for stripe rust on susceptible cultivars has not been applied, remember that economic benefits from spraying after mid-flowering decrease rapidly, particularly with crops of low potential yield. Farmers are reminded of the 49-day withholding period. • Grain aphid numbers are increasing. Regularly check the' base of developing seed heads. If an average of five aphids a head are counted an aphicide could be worth while. See AgLink FPP 509. BARLEY

• A lot of crops have plants with bright yellow leaves. They could have barley yellow dwarf virus (8.Y.D.V.) or be exhibiting nitrogen deficiency. If the affected plants are in patches suspect B.Y.D.V. If a large area is affected it is more likely to be a temporary nitrogen deficiency.

• Watch out for a brown netting or crosshatch effect on leaves caused by net blotch fungi. If severe a spray of Mancozeb or Tilt may be advisable. (Tilt is not registered for use in New Zealand.) • Spraying for cereal aphids is now not considered worth while but rose-grain aphid populations should be monitored.

• Some crops are showing unusual symptoms on their leaves. These range from a few dark spots to large brown blotches consisting of a central dark spot surrounding a halo of smaller blotches. No pathogens have been detected and the cause is tnought to be the geneticexpression of extreme nu-

trient changes or environmental conditions. LUCERNE

• Both bluegreen and pea aphid populations are decreasing although pea aphid in particular could still be a problem. Ladybirds and parasites introduced by D.S.I.R. are helping to decrease aphid numbers so spraying could be deferred, provided the crop is regularly checked. LOTUS SEED CROPS • “Potato” bug numbers are building up. But don’t apply an insecticide yet. Wait for a couple of weeks and watch Plant Health Monitor for further information. CLOVER SEED CROPS • Clover case bearer moth numbers are building up. Sample crops very regularly from now on, see AgLink FPP 620. • Not all springtails cause damage. Don’t spray to kill the small, black elongated springtail — it feeds entirely on plant litter. The small orange globular springtails may damage clover, but only at very high densities. The larger hottie green globular “lucerne flea” springtail can cause damage and growers should watch out for this pest in particular. Ring AgPhone 66-887 for more details. • Sample flower heads for aphid populations. If an average of five aphids/ flower is seen a spray could be worth while. The use of bromophos or diclorvos will control both this pest and clover case bearer. Be very careful how you use such insecticides; they will kill bees if not applied properly. OATS • Rose-grain aphid populations continue to build up. Provided the flag leaf is not infected • spraying can be deferred.' BRASSICAS • Continue to check for aphids especially on the later sown crops. Many crops are suffering from moisture stress and aphid infestations can depress dry matter

yields even more by causing leaf distortion and possibly introducing virus. PEAS • Check for wild oats but only spray at the correct growth stage. See AgLink FPP 296. TREES AND SHRUBS • Adult grass-grubs are still causing leaf stripping. Damage will continue for at least another 3-4 weeks especially in the higher altitude areas of Canterbury. APPLES • Growers are warned of a possible problem with capsid bugs. Watch for dimpled misshapen fruit. Send samples to the Plant Health Diagnostic Centre (Lincoln) for checking. • An insecticide to control European red mite will be required next week. Listen to the radio for details or consult’ .your horticultural advisory officer. TOMATOES • Bacterial diseases seem common in glasshouse crops this season. Growers should be aware of the symptoms of stem bacteriosis, canker and speck. Affected plants should be removed rapidly. See AgLink HPP 72/11. Attention to hygiene is very important. • White fly is still a potential problem. Growers should continue with control measures. HORTICULTURAL CROPS • Many crops have been affected by frost and hail recently. Growers with affected crops especially berryfruits are reminded to keep up regular Botrytis control sprays. • Leafspot diseases are now starting to appear especially in blackcurrants and celery. • Aphid populations seem to be particularly bad right now on many crops. Regular checking, even after spraying. is recommended. • Spittle bugs are still around. For the latest control strategies consult your local horticultural advisory officers.