Blue Prune Drop

Blue Prune Drop and Leaf Scorch

Blue Prune Drop

July 4, 2013

Blue Prune Drop and Leaf Scorch in Glenn and Tehama Counties

According to Bill Krueger UC farm advisor emeritus, Glenn County and Richard Buchner UC farm advisor, Tehama County, overheated prunes are succumbing to pressure due to high temperatures over the last few weeks. Blue prune drop and, in some cases, an associated leaf scorch, often develops following the rapid onset of high temperatures as occurred in June of this year.
Damaged prunes color prematurely (turn blue) and usually drop from the tree. The sun-exposed fruits on the top or south side of the tree are more likely to be affected by becoming sunken or flattened. Leaf scorch and dieback may develop in leaves and twigs near the damaged fruit. When damaged leaves dry, the veins may be a darker brown than the rest of the leaf.
Blue prune  drop is associated with heat stress. Excessive heat results in damage to the fruit that is thought to produce a toxin which is transported to spurs, leaves and shoots resulting in the leaf scorch symptoms. Leaf scorch symptoms are always associated with damaged prunes. They do not occur in areas of the tree with no fruit or on young trees without a crop. Anything affecting fruit temperature can have an effect. This would include:
1. Irrigation – Drop and particularly scorch are generally more severe on shallow soils with limited water holding capacity or in orchards toward the end of their irrigation cycle at the onset of heat. Adequate soil moisture insures maximum evapotranspiration and cooling of the plant.
2. Tree Position or Fruit Location - Leaf scorch is usually worse on overheated border trees, or on the south side of individual trees with greater sun exposure.
3. Cultural Practices – Blue prune appears to be less severe in orchards with cover crops than in clean tilled or drip irrigated orchards. Transpiration from an adequately irrigated cover crop should contribute to orchard cooling. In addition, a vegetated orchard floor reflects less sunlight than dead vegetation or bare ground.
4. Nutrition - While blue prune and leaf scorch does not appear to be directly related to potassium deficiency, anything adversely affecting tree health and condition could contribute to higher fruit temperatures. Adequate tree nitrogen levels promote vegetative growth that shades fruit from direct sunlight.
Krueger and Buchner report they have no sure ways of preventing blue prune drop and the associated leaf scorch. However, you can reduce the risk by making sure trees are healthy, vigorous and well supplied with water. Because the damage is caused by heat and not a disease, it should not continue to expand in the tree. Damaged wood should be pruned out during the dormant season.
Source: University of California Division of Agriculture and Natural Resources Cooperative Extension 

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