Walnut blight is caused by bacteria that live over the winter in dormant buds. As the shoot elongates, the bacteria are distributed along the emerging shoot and flowers. Rain spreads the bacteria and aids in the infection process. Control depends on the timely application of protective copper sprays to the developing nuts before rain events.
Spray timing will vary among orchards depending upon the history of disease and weather conditions each year. Rain and temperature are the driving environmental factors in disease development. The ideal temperatures for blight development are between 54 and 63 degrees, which usually occur in April and May.
Copper-maneb sprays provide better and more consistent control than fixed copper sprays alone. Check with your Ag Commissioner’s office to see if a Section 18 allowing Manex use is in effect for your county.
Research on the appropriate initial spray timing is ongoing. At this time, it appears the first spray should be applied sometime between bud break and the first pistillate bloom. The earlier timing looks suitable in orchards with histories of blight during rainy weather. Bud break is when the tip of the shoot has emerged about an inch, but before the leaves unfold. Estimate bud break within the orchard and not on the outside trees.
Treat when there are enough buds open to make it economically feasible. If rain is expected, it may be economical to spray at 10 percent to 20 percent bud break in orchards with a history of blight. If no rain is expected, spray between 30 percent to 40 percent bud break and first pistillate bloom.
The pistillate flowers are the small nutlets that form after a few leaves emerge. Apply subsequent sprays before rain for best results. In orchards with histories of walnut blight damage, protective treatments at 7 to 10-day intervals during protracted spring rains are necessary for adequate protection.
• Codling moth
Hang codling moth pheromone traps in the southeast quadrant of the trees in mid-to late-March to detect first moth emergence. Traps placed high in the tree canopy catch more moths, which is particularly useful in orchards with low populations. In the spring, the biofix is the date when both consistent trap catches and sunset temperatures above 62 degrees occur. Begin accumulating degree-days on this date using a lower threshold of 50 degrees and an upper threshold of 88 degrees.
Degree-days help growers predict codling moth development and more accurately time spray treatments. Monitor orchards individually because the codling moth emergence date and activity will vary among orchards. Mating disruption should be in place before moths start emerging. Specific details on calculating degree-days and more in-depth information on managing codling moth is available on the UC IPM Web site at http://www.ipm.ucdavis.edu.
Consider codling moth damage history over the last several years when developing a management plan. Growers who estimate codling moth damage by cracking out harvest samples have an advantage over those that rely on grade sheets because the latter report combined damage from both codling moth and navel orangeworm.
Infestation history and field observations along with trap count numbers, determine how aggressive management strategies should be. A program implemented in an orchard with substantial damage last year and high trap counts this year will differ from one where little or no problem exists. Tailor your program to fit the situation in your orchard.
• Ground squirrels
Controlling adult ground squirrels before they reproduce in the spring is a critical part of good management. Burrow fumigation is the method of choice at this time of year when squirrels feed on green vegetation and are not interested in baited grains.
A fumigant program followed by anticoagulant baiting in the summer can control 90 percent of the population. Squirrels typically breed from late January to early March, but the time can vary with the weather and location. During the winter, squirrels with good fat reserves hibernate in sealed burrows. Other squirrels, usually the spring born juveniles, forage above ground even in cold weather.
For the best results, use burrow fumigants about three weeks after the first squirrels emerge from hibernation. Zinc phosphide tablets, a restricted use material, is an effective substance. Save supply and labor costs by treating only active burrows. Cover burrow openings and treat only those that are re-opened. Fumigate following rain or irrigation since soil moisture is necessary to release the gas.
The best timing is early morning or evening when ground squirrels are most likely to be inside the burrows. Check all treated burrows a few days after fumigation and treat any that have opened. Do not treat burrows that are near or under buildings. More information can be found on the UC Ground Squirrel Best Management Practices Web site at http://groups.ucanr.org/gsbmp.
• Check stakes
Limb or trunk damage from rubbing against stakes is very common and negatively affects growth and scaffold development. Prevent damage and economic losses by inspecting trees and removing or pulling stakes away from trunks and limbs.
• Newly planted tree care
Paint the trunks of newly planted trees with white interior latex paint to prevent sunburn. Be sure to paint to ground level; new trees usually settle leaving a small, unprotected area at base where temperatures are the highest. When staking is necessary, angle stakes slightly into the wind about 8 to 12 inches away from the trunk. Tie loosely to avoid shoot and trunk injury; this also allows the tree to move with the wind increasing trunk size.
Zinc foliar sprays are most effective in correcting deficiency when applied to new spring shoot growth. Zinc can be included with most copper materials (check the label). The effectiveness of zinc foliar sprays drops off significantly later in the season when the cuticle thickens.