Plant health monitor

Press, 3 February 1984, Page 25

Plant health monitor

Cereal harvesting is now under way in many areas, so the importance of insect and pathogen problems has declined. However, growers should use this season’s disease and pest experiences to start planning appropriate crop hygiene measures and paddock rotations for the future.

From now on pasture and winter green feed pests and diseases will become more important. Remember to check carefully both above and below ground before sowing and obtain information on potential problems to look out for after sowing. Cereals

• Several unusual cases of grassgrub damage to mature wheat have been reported recently. Because no prior damage was recorded, this damage was most likely caused by third instar larvae from very early adult flights last spring. No control is possible for the wheat crop, but beware of damage to autumn sown pasture or winter green feeds.

• Leaf spotting on barley is prominent in many crops nearing maturity. Many farmers, believing this could have been caused by a pathogenic fungus, have applied a broad spectrum cereal fungicide. Treated crops have remained much greener than untreated ones, but the spots have continued to develop.

This suggests the problem is one known as physiological leaf blotch, which is not caused by a pathogen. The response to spraying is possibly a result of two effects. The fungicide may have stopped the development of fungi that live on dead or dying tissue which accelerated leaf yellowing and death. Fungicides may also stimulate plant growth temporarily, even in the absence of a pathogen.

• Note the incidence of take-all, eyespot or footrot in barley and wheat crops and use this information for crop sequence planning to avoid major losses next year.

• Stripe rust in wheat heads is causing concern. It will result in some shrivelled grain. No control is possible once established in the heads, but note that early effective rust control on foliage normally results in healthy heads. Crops • Start planning now to control the more difficult perennial weeds such as yarrow and couch, especially in white clover crops. Plan for an autumn-winter fallow followed by spring sown crops. As soon as the crop has been harvested, and perhaps grazed, allow the paddock to freshen up then apply four litres of glyphosate (Roundup) per hectare.

Leave for about three weeks than chop by discing or rotary hoeing to stimulate remaining root growth. Recultivate as new shoots appear. An alternative couch control is to rotary hoe at least twice followed by other cultivation as appropriate. The critical point is to rotary hoe or cultivate before the five-leaf stage when new rhizomes will form.

Brassicas. Beware of winter brassica establishment problems caused by pathogens (wire stem and “damping off’) and insects (Nysius springtails and grassgrub). Growers can either treat the seed themselves with captan and lindane before sowing (consult your local M.A.F. adviser) or use com-mercially-treated pelletted seed. Fruit

• Red mite populations on apples in the Loburn area are in the egg stage now, but hatching will occur soon, so a spray may be needed in about a week. In other areas growers should check both apple trees and currants for mite populations. Where necessary, time spraying for just after egg hatch. Remember to check for predacious mites. If one or more predacious mite is seen for every seven to 10 red mites then no spray is required. Consult your local horticultural adviser for more details. • The crawler stage of scale insects on pipfruit has been noted recently. Where there has been a history of scale problems, a spray such as Lorsban could be applied now. Check first, as scale population development has been delayed this season. Pasture • Preliminary counts of porina using the board technique have given variable results so far. Densities have ranged from nought to 58 per square metre with the highest numbers recorded on irrigated pasture. Caterpillars are about 200 mm long at present. Pasture damage will start to accelerate during February, but could be masked if the present growing conditions continue. • Some quite high grassgrab populations have been recorded recently, especially under irrigated pasture. A significant number are already in the third instar stage and feeding voraciously. It appears grassgrub may again be a problem in South Canterbury but less so in coastal and the Mid Canterbury plains.