We are noticing more dieback of lower limbs in many almond orchards this year. This “disease” has become quite significant, killing a large percentage of the lower wood in some orchards. Padre appears to be the most seriously affected variety, although Butte can also be very bad. Nonpareil, Carmel, Aldrich and a few other varieties are affected to a lesser degree. The problem appears to occur primarily in orchards older than 10 years.
The problem is associated with weaker, small diameter wood in the lower canopy, although shoot death sometimes extends 10 feet or more from the ground. Beginning in late April, leaves on affected limbs first turn yellow, then brown as the limb collapses. Limbs may die right up to point of attachment but the large wood of the scaffolds appear to remain unaffected. If the bark on yellowing limbs is scraped away with a knife, you can often see brown spots underneath. These spots seem to grow together to form large dead areas which then causes the whole limb to collapse. Shoots continue to collapse throughout the summer. Sometimes darkened cankers can be seen extending deep into the middle of the branch if you cut the branch in cross section. Sometimes the cankers are wedge-shaped, sometimes they are not.
With help from University of California plant pathologist, Themis Michailides, we surveyed several orchards in Stanislaus and Merced counties to determine if the limb death is a disease or if it is caused by some other problem. Clearly the limb death is not caused by excessive shading, anthracnose, Alternaria or bread mold. In all surveyed orchards, two different species of fungi were consistently found growing on affected limbs. The fungus most consistently isolated from “diseased” limbs was an unidentified species of Phomopsis. There are many species of Phomopsis that cause canker diseases in grapes, figs and other plants. Phomopsis amygdale causes limb dieback of almond in some Mediterranean countries, Australia and in South America. This fungus was also shown to be the cause of a fruit rot and associated limb dieback in a Butte County almond orchard in 1998 after an unusually wet spring.
The second fungus commonly isolated from affected shoots was Botryosphaeria dothidea. This is the fungus that causes band canker, a fairly rare disease affecting the trunks and scaffolds of young almond trees. It is also the cause of panicle and shoot blight of pistachio, a serious disease for that industry. Recently, this fungus has been shown to cause shoot dieback in local walnut orchards. During our survey, we found this fungus sporulating on dead walnut shoots in orchards next to affected almond orchards. We also found spore-producing structures in nearby cedar and redwood trees. Botryosphaeria fruiting bodies were rarely found on the affected almond wood. It is unclear at this time if one or both of these fungi are responsible for this lower limb dieback. If these fungi are involved, it is unclear when infection is occurring.
This spring, we tried to reduce lower limb dieback in a badly affected Butte/Padre orchard by spraying Abound fungicide from petal fall through June 1. Of course, we would never want to apply any fungicide that often back to back in the real world, but we were just trying to determine if the problem could be reduced with spring-time fungicide sprays. Unfortunately, even trees sprayed four times after petal fall had just as much lower limb dieback as the unsprayed trees. Abound should have been very effective against both of these fungi. This information suggests that infection had occurred prior to petal fall and that it may take several months for the dieback symptoms to show. In fact, the literature from other countries suggests that Phomopsis infections on almond shoots occur primarily in the fall. In the coming months, we will try some fall and/or dormant treatments to see if we can reduce this lower limb dieback problem. However, based on conversations with growers, typical dormant copper applications do not seem to make a difference.
We are just in the first stages of figuring this problem out. At this point, I suggest pruning out affected and dead limbs before the fall rains arrive (and while trees still have leaves so you can identify which limbs to remove). Removing these limbs should reduce the inoculums in the orchard. It is also important to keep limbs strong by keeping scale populations low with an occasional dormant oil application. We will continue to work on this problem and hopefully come up with some answers for next season.