Tests continue to look for answers on lower limb dieback

Over the past couple of years, many orchards in California have been affected by a disease called “lower limb dieback.” Padre and Butte are affected most severely. Symptoms begin to show in late April or May as leaves on small, lower limbs turn yellow. Over a period of weeks, the limbs become girdled from enlarging cankers and die. Symptoms progress through most of the summer.

Two canker-forming fungi, Botryosphaeria dothidea and Phomopsis sp., are consistently isolated from cankers on dying limbs. However, it cannot be said for certain that these fungi are the primary causes of the disease as they may come in as secondary invaders. These fungi from dead limbs have been isolated in nearby walnut orchards as well as landscape trees like cedar and redwood. These fungi sporulate profusely on these alternate hosts but don’t appear to sporulate nearly as much on infected almond wood. It’s suspected that most almond infections are from spores that are blown in from outside of the orchard.

Last spring a test tried to reduce lower limb dieback in a badly affected orchard by applying fungicides from petal fall through early June. Unfortunately, these spring fungicide applications had no effect on reducing symptoms last year.

Based on reports out of Europe that Phomopsis infections may occur primarily in the fall, field trials were conducted in three orchards testing fall treatments. In two orchards, the growers applied copper hydroxide (Kocide DF at 12 pounds per acre) or liquid lime sulfur (15 gallons per acre) in mid-late October. In a third orchard, several other treatments were tried in smaller plots using a handgun sprayer. These treatments included Kocide DF applied in October and December, liquid lime sulfur in October and December, Pristine fungicide (14.3 oz per acre) applied every two weeks from Oct. 14 through November followed by an early December Kocide application, NutriPhyte P (0.5 gallons per acre), and PlantShield, a commercial formulation of Trichoderma harzianum, which is a biological fungicide.

The bottom line is that no significant reduction was detected in lower limb dieback symptoms this spring by any of the fall treatments in any of the orchards.

This was, of course, very disappointing. There may be a few explanations for our poor results. First, many old cankers that had “died out” last summer reactivated this spring leading to more limb death. Of course, a fungus that survives from one season to the next inside of a limb will not be affected by a fungicide spray.

Another possible reason tests did not reduce lower limb dieback significantly with the fall treatments is because the two fungi can sporulate and infect new wood at a very wide temperature range. Sporulation of Phomopsis amygdali pycnidia occurs from 34 degrees to 100 degrees and infection can occur from 41 degrees to 97 degrees as long as there is moisture present. This means infection could potentially occur any time from the first rains in the fall through the last rain event in the spring. There is no practical way to protect trees with standard fungicides for a period of six to seven months or longer. It is also possible that there is another primary cause of the disease we have yet to discover.

One out of the three cooperating growers pruned out all the dead and diseased wood in his orchard last fall. This orchard had significantly less dieback this year than last year and also much less than the other two orchards that work was conducted. It’s assumed that this is because the grower removed limbs containing old cankers, which would have reactivated this spring.

Management of this disease may require removal of diseased limbs in combination with multiple applications of a long residual fungicide like copper. It may be best to prune out the wood during the summer while it is easy to identify affected limbs. More work will focus on this problem that will hopefully result with clearer management suggestions.

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