In a year when heavy rains occur during the bloom period, it is not unusual to find evidence of jacket/green fruit rot on almonds. It is caused by Botrytis or Sclerotinia infections, usually of the senescing petals or floral jackets shortly after bloom. El Niño rains in February were frequent during the almond bloom of 2016, and as a result, many growers reported above normal amounts of jacket rot, even in well sprayed orchards with properly timed fungicides.
This problem originates when clusters of flower petals and the splitting floral jackets become lodged in dense blooming spurs, and do not fall from the tree. When this happens, this mass of decaying tissue spreads onto adjacent plant parts and can cause developing nuts to rot and even the death of whole spurs. This is especially true of prolific blooming varieties such as ‘Butte,’ ‘Wood Colony,’ and ‘Independence,’ to name a few.
During 2016, many orchards were in full bloom when they received long periods of light drizzling rain with little-to-no wind. This resulted in many petals and jackets remaining in the spurs.
An Unusual Twist
An unusual twist to this “typical” jacket rot was quite common this year. Numerous leaf lesions were showing up in late March that had large infected areas that appeared from a distance to look similar to the Alternaria lesions seen in mid-summer, only larger.
Closer inspection of these blighted leaves indicated that they were being caused by rotting petals stuck on the leaf surface, detached pistils or rotting jackets leaning on the leaves.
None of these appeared to be primary infections and the necrosis appeared to be caused by the rotting tissue touching it or toxins created by the rot.
Was This Preventable?
Regardless of which decaying plant part was responsible for the leaf injury, much of this was unavoidable, with the worst cases being the orchards that did not receive protective fungicides. The extent of jacket rot present throughout the trees made the incidence of leaf blighting greater than normal, prompting concern from many growers.
There are certainly other potential causes of leaf injury at this time of the year such as scab, anthracnose, Pseudomonas, nutrient spray burn and herbicide drift, or co-distillation, but a close examination of this leaf injury will show the root of its cause originated from the jacket rot.
In the big picture, the number of actual leaves affected on an individual tree is not very significant. The fact that their damage is vivid due to the discoloration of the tissue and the distribution throughout the canopy is what makes it so dramatic. It actually is more of an indication that crop loss due to nut damage has occurred.