Several calls have been coming in over the past few weeks regarding unusual leaf symptoms on almonds. For the most part, the symptoms have appeared across varieties, but with one variety being more heavily affected within a single orchard. Symptoms also have appeared in waves, starting about mid-April, again in May, and then again in June. Orchards all over the San Joaquin Valley have been calling in with reports, varying in ages, varieties, nutritional status, and orchard spray practices.
The symptoms appear to start off as small, yellow pin-head sized lesions on the leaf (Figure 1). Following a few days of heat, the lesions enlarge, and become necrotic, often abscissing from the leaf (Figure 2). The margins of the leaf appear to be the most severely affected, with some damage evident throughout the leaf blade.
I initially thought that the problem was due to foliar nutrient sprays – either due to contaminated products or cation activation (iron, zinc, manganese, magnesium, etc.) by a rain event. This didnt seem to fit the symptoms in every orchard due to some orchard owners not spraying foliar nutrients.
Symptoms also appeared similar to potassium deficiency, but the widespread occurrence throughout the valley does not support a nutrient deficiency.
The possibility of corky spot was brought up, especially since the leaf symptoms look similar to the pictures of this disease. Corky spot is an unknown disorder that tends to be associated with nonpareil planted on seedling almond type rootstock (i.e. bitter almond). Due to the widespread occurrence of this symptom across differing orchard ages, nursery sources, and rootstocks, I doubt that this is a case of corky spot.
I also was concerned about some type of foliage disease due to the spread of the disease and season progression. Samples were sent to UC Plant pathologists and they were unable to isolate any disease causing agents. Inconsistent isolation of various fungi from different samples led to the conclusion that the disease was not due to a fungal pathogen.
Through discussion with other advisors across the state led to the possibility of Peach Silver Mite (PSM), a mite of the Eriophyid mite family. Damage observed from previous years in Sacramento valley appears to be similar to the damage observed this year. PSM feeding and population growth occurs during cooler temperatures. Upon hot temperatures, the mite basically disappears from the orchard. PSM are quite small, need a 15x or stronger hand lens to see, and are controlled by most miticides, oil, and even sulfur. Observations by UCCE advisor Franz Niederholzer in Yuba/Sutter Counties indicate that they are not controlled by Acramite.
Experience from other advisors suggest that this damage is due to PSM or some other mite within the Eriophyid family. This year, many people applied miticides later due to the cooler temperatures and reduced pressure. This may have allowed the populations enough time to build to high levels within the orchard and cause the observed damage. Damage that is appearing now most likely occurred a few weeks ago. For the most part, there is nothing for the grower to do at this point since the damage already occurred. I don’t think, unless we have another cool year, that these symptoms should appear next year.