Abamectin

AS WE enter the month of August, growers will quickly learn what kind of year 2005 will be for shaking. As a rule, when sets are not heavy, nuts do not shake as easily. This is particularly true of Nonpareil, Mission, and Padre as well as others. The causes of this condition are not well understood. If hull rot damage or gumminess due to diseases such as shothole or boron deficiency are present, knocking will be more difficult.

In years where shaking is difficult with light crop sets, differences in cultural practices do not seem to point towards a particular practice as the cause of the problem. Orchards that are well irrigated knock similar to orchards that are underirrigated. Trees with excellent nutritional status have difficulty knocking as do trees with nutrient deficiencies (boron excepted). Heavily pruned trees are as difficult to knock as lightly pruned trees. Low-density orchards and high-density orchards can be equally troublesome to knock.

In general, trees that are harvested when nuts are too green or too dry are the most difficult to knock. Increased shaking force will often result in bark damage. When all nuts have achieved hullsplit and the majority of split nuts are beginning to dry, shaking is usually best. This means the nuts will need to lie on the ground more days to dry.

This brings up a Catch-22 regarding insect pest management. There is good evidence that a prompt harvest when nuts are mature will minimize the risk of damage from the third generation of navel orangeworm (NOW). However, the increased drying time puts one at greater risk of suffering economic ant damage. Depending on which pest is the most significant in an individual block, control strategies will vary. If ants are a problem, they need to be addressed regardless of harvest timing.

There are several chemical strategies available that are effective for ant control, with baits being the most effective. The choice of material will affect optimum treatment timing. Esteem (pyriproxyfen, Valent USA) bait should be applied a couple months before harvest. Clinch (abamectin, Syngenta Crop Protection) bait is effective about a month before harvest.

For more information on managing navel orangeworm, see the University of California IPM Pest Management Guidelines at www.ipm.ucdavis.edu/PMG.

Harvest Quickly

With ants under control, a prompt harvest will minimize exposure to NOW. Keep in mind that a prompt harvest is only the first step in harvest time NOW management. The nuts need to be swept, picked up, and hulled and shelled as quickly as possible. If delays are expected, or stockpiling is used, the crop should be tarped and fumigated as soon as possible. If a block has had a history of NOW damage, it means that there is probably a history of overwintering mummy nuts. This is the primary way that NOW populations are sustained within a given block.

Growers may cite wet and heavy soils prohibiting shaking or labor costs of poling as rationale for failing to do winter mummy sanitation. If NOW is the major pest in a particular block, then these reasons lack legitimacy. University of California studies have shown that as few as two overwintering nuts per tree can lead to economic NOW losses. There are orchards that regularly support 10 to 20 mummies per tree each winter. Hull split sprays at best reduce the potential damage by only one half. Cultural management needs to be incorporated into any control strategy.

Wesley Asai is a former University of California pomology farm adviser and is currently owner of Wes Asai Pomology Consulting in Turlock, CA. E-mail questions or comments about this article to wesasai@aol.com.

Copyright Meister Media Worldwide Aug 2005
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