Plant health monitor

Press, 2 March 1984, Page 20

Plant health monitor

Crops have now been harvested in most areas and yields of 7 to 8 tonnes per ha have been quite common, according to the M.A.F. These higher yields are generally the result of growers paying attention to details of crop management, especially nitrogen application and disease control.

With the cropping season almost over, emphasis is now placed mainly on insect pests affecting pastoral farms. It is now confirmed that porina damage is going to be a serious problem in many areas this winter. Pipfruit

• Many leaf roller moths are flying and laying eggs. Prepare to apply spray of chlorpyrifos (Lorsban) or azinphos methyl (Gusathion) to coincide with maximum egg hatch in about two weeks. If scale insects or mealy bugs are present, Lorsban is the preferred insecticide.

• Two-spotted and European mite eggs continue to hatch. Monitor plants by checking leaves with a hand lens to determine when about 50 per cent of the eggs have hatched. This season it is unlikely that many pipfruit, stonefruit or currant plantings will require a miticide. However, if considered necessary, now would be a good time to spray. Brassicas

• Damage by the Brassica leaf miner is evident in many crops. These insects usually damage the older leaves which die prematurely. Crops will compensate for this damage so control measures are usually unnecessary. Greeufeed ryegrasses • Argentine stem weevils will stop laying eggs soon and significantly reduce their feeding rate. From March onwards it will no longer be necessary to use insecticide on sowings of short rotation ryegrasses. Cereals

• Blackening of both wheat and barley heads has been noticed recently. This is being caused mainly by secondary saprophytic fungi. They are not usually pathogens but at high levels can cause “black point.” Control is not considered

economic. • Be prepared to check all stored grain in about a month for stored insect and mite pests. Early fumigation may be required if there are signs of insect or mite build-up. Any grain stored at 15 per cent moisture (or higher) should be checked even sooner.

Grapes • All Canterbury and Otago grape growers should continue to be concerned about the discovery of phylloxera in Marlborough. This root feeding aphid is nearly impossible to control once established and seriously affects vine vigour. All new plantings must be with resistant rootstock to remove the risk of infestation. Any other rooted material must be regarded as phylloxera-susceptible so growers should take precautions to prevent importing the pest on to their properties.

Remember that any plant material or used machinery from phylloxera areas can introduce the pest. Pasture

• Reports of Tasmanian grassgrub adult flights have been reported over the last month. Light land farmers should know how to identify the black headed grubs. Check light land permanent pastures after the next significant rainfall. Look for small mounds of fluffy soil casts caused by larval activity. The casts are not webbed together like porina casts. Note the location of any Tasmanian grassgrub populations for later sampling to predict late winter pasture damage. Be sure also to check the same area

soon for both grassgrub and porina.

• Counts of porina from many areas now confirm that pasture damage, especially on light land, will be severe this winter. Some farms have an average of 40 per square metre over all pasture. Spray-irrigated pasture is also harbouring high populations but those on flood irrigated pasture are generally low.

Farmers should be preparing to control potentially damaging populations during March by grazing down infested pasture now to ensure adequate spray penetration and coverage.

• Be prepared to sample any brown patches that become visible after the next significant rain. Check these areas for grassgrub. Make sure that mixed populations of grassgrub and porina are recorded. If about 150 grassgrub per square metre are sampled then as a rule halve the porina economic thresholds. See your FAO for details.

• Don’t carry out spraying for scrub weeds if dry conditions are maintained.

• For cosmetic effect only, both Californian and Nodding thistles can be topped now before spraying M.C.P.B. in autumn.

• This should be a good season to reduce barley grass and thistle invasion. Maintain pasture density during the late autumn by careful grazing management over the next few weeks. If the pasture is dense enough in late autumn, it should prevent barley grass and thistle seedling survival.