It is too early to tell if a crop is set in Sacramento Valley prune orchards, but it's not too late to plan ahead to feed and protect a good crop. If I were a grower, here's what I'd do to give myself the best possible chance of making money growing prunes in 2009.
• Don't over crop the orchard. Too much fruit on a tree loses money. Excess crop load reduces fruit sugar and increases dry away. The risk of branch dieback from potassium deficiency increases with crop load.
Too much fruit weakens the tree. Weak trees risk a light crop next year. Strip and count fruit per tree in late April/early May to measure crop load. Shaker thin, if needed, to remove excess fruit.
• Irrigate as needed. Fruit end cracking occurs when water stressed trees are irrigated. The risk of end cracking is highest in May, June, and early July. To avoid end cracking, irrigate to meet tree water needs at least through the end of June. Test soil moisture to find out if the orchard soil is drying out, or use the "pressure bomb" to determine tree water status.
To save drying costs (improve dry away) cut off water as early as possible before harvest. The cutoff date in your orchard will depend on soil conditions and irrigation system.
• Fertilize to feed the crop and keep leaves on the tree. Prune trees need potassium (potash) and nitrogen fertilizer to feed a good crop. The more fruit, the more fertilizer needed. Potassium nitrate sprays help avoid potassium deficiency.
Prune trees need around 100 pounds of actual nitrogen per acre per good crop year. In a light crop year, you might get away with applying no nitrogen. However, be careful. Low nitrogen orchards are more vulnerable to bacterial canker infection. A foliar zinc spray may be needed every-other year or every year. Take a leaf sample in July to see how well your fertilizer program is working.
• Manage pests. Keep leaves on the tree to grow the sweetest, biggest fruit possible.
• Prune rust can defoliate trees resulting in less fruit sugar and reduced dried fruit size. Look for prune rust spots once every week beginning May 1. Spray sulfur when the first rust spot is found. Repeat sulfur application if more spots are found. Don't apply sulfur if rust is not found in the orchard. (Sulfur can harm "good" mites that eat spider mites.)
• Spider mites can also defoliate trees and reduce fruit sugar levels. Spider mite numbers can double in one week of 100 F weather. Look for spider mites once a week in the orchard beginning June 1. (Start scouting earlier if it is a dry spring.) Treat if a significant number of spider mites are found and mite predators are absent.
• Fruit brown rot can damage fruit as harvest approaches. A fungicide spray can help control fruit brown rot. Spray fungicide one to two weeks before harvest if wet weather is forecast or orchard has a history of fruit brown rot problems.