Franz Niederholzer1 CommentSeptember 26, 2013Diseases

Almond Leaf Scorch

Written by Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties and Joe Connell, UC Farm Advisor, Butte County In the last month, several PCAs and/or growers have reported trees in almond orchards in the south Sacramento Valley with symptoms similar to those of almond leaf scorch (ALS) — a disease caused by the bacterium Xylella fastidiosa. Five of the six fresh leaf samples subsequently submitted to a lab for ALS evaluation tested positive for Xylella fastidiosa. Varieties that appear more susceptible in the field include Peerless, Sonora, Winters, Livingston, and Wood Colony.

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Roger Duncan13 CommentsJune 26, 2013Diseases

Bacterial Spot – A New Disease of Almond in the San Joaquin Valley

Written by: Roger Duncan, Brent Holtz, David Doll and Themis Michailides Earlier this spring, we received reports from growers and pest control advisors that they had observed a few San Joaquin, Stanislaus and Merced County almond orchards with large amounts of amber-colored gum balls exuding from the hulls.  The damage has been predominantly on ‘Fritz,’ but there are reports of similar damage on ‘Monterey’ and ‘Padre.’  In some orchards, Fritz is severely affected while the Nonpareil and other pollinators are very clean. 

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David Doll6 CommentsJune 6, 2013Diseases

Bacterial Spot on ‘Fritz’

Written by Brent Holtz, David Doll, Roger Duncan, and Themis Michailides We have visited and received samples from orchards in the counties of Merced, Stanislaus, and San Joaquin Counties that have been experiencing symptoms of amber colored gum exuding from almonds. The damage has been predominantly on the variety ‘Fritz,’ but reports are coming in of similar damage on ‘Monterrey’ and ‘Padre.’

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Brent Holtz5 CommentsJune 22, 2012Diseases

Perennial Canker of Almond

Written by Brent Holtz, UCCE San Joaquin County Symptoms of perennial canker of almond. Photo by F. Niederholzer. Perennial Phytophthora Canker has killed many almond trees. Two plant pathogenic fungi, Phytophthora cactorum and Phytophthora citricola are primarily responsible. This disease is lethal, perennial, and is not associated with pruning wounds which differentiates it from “Pruning Wound Canker.” Perennial Phytophthora Canker kills almond trees by girdling the scion and is usually initiated in a conducive environment either below the soil surface or on the tree trunk where the trunk and branches join. Phytophthora cactorum is usually associated with infections initiated near the soil surface, while Phytophthora citricola typically causes aboveground infections initiated near tree branch crotch pockets. Dr. Greg Browne, USDA-ARS, has found that the source of inoculum for above ground infections is likely debris from the orchard floor containing fungal spores that are blown onto trees during harvest. This debris and spores are most likely washed off the tree during rains, with some accumulating in main-branch crotch pockets or depressions where main branch scaffolds join the tree trunk. These scaffold pockets containing soil, water, and fungal spores most likely offer a conducive environment for Phytophthora infections to take place. Dr. Browne’s research has provided almond growers with some control measures that can be taken to prevent Perennial Phytophthora Canker. He has shown that phosphonates (e.g. inorganic and organic salts of phosphonic acid) have provided systemic activity against a number of diseases caused by many of the Phytophthora species of plant parasitic fungi. The phosphonic acid derived from phosphonates probably disrupts Phytophthora growth and appears to intensify almond tree defenses against infection. Fortunately, the phosphonate can be translocated both upwards and downwards in the tree due to its mobility in both xylem (water going up) and phloem (sugars going down) tissues.

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David Doll0 commentsJune 9, 2012Diseases

Summer Disease Control: Should I Spray Again?

This past week, rain showers hit across the state, extending as far south as the Madera/Fresno area. In the Merced area, rain was variable, with most reports indicating about a 0.5″ of rainfall.I have had a few questions regarding the the need for another fungicide spray for the summer diseases of Rust and Alternaria. The following conditions and considerations should be kept in mind when deciding the need for another spray: Orchard History. If defoliation occurred last year due to these pathogens, there is a good chance that the over-wintering inoculum levels are high, increasing the chance for an epidemic this year. This factor remains important even if sprays were applied earlier in the season. Conditions post rain event. The hours and days after the rain were windy and warm, which dries the orchard. These conditions helped reduce the duration of leaf wetness, and canopy humidity, creating an environment that would be less favorable for disease. Timing of last fungicide spray. If a fungicide was applied in late May due to a delay on the “May Spray,” there is a good chance that there is enough residual control of the material. Sprays of DMIs and strobilurins provide protection of covered tissue for 14-20 days in conditions without rain. If a fungicide hasn’t been applied since the late April/early May, a spray should be considered due to the lack of leaf protection. Any new growth that occurred since the last spray will not have adequate fungicide protection. Tolerance of disease. My perspective in managing foliar diseases is to achieve the goal of having active leaves present on the tree in late October. In other words, there can be some disease present on the trees, but the tree has not defoliated and still has the ability to photosynthesize. Many people have a different threshold, which usually

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David Doll0 commentsAugust 26, 2011Diseases

Alternaria Found within Merced County

Relatively rare in the northern San Joaquin Valley, I was in an orchard today with Alternaria Leaf Spot. Caused by the pathogen Alternaria alternata, this disease usually appears by mid summer and causes defoliation. Significant problems with Alternaria have been observed in the southern San Joaquin Valley and northenr Sacramento Valley. In some of these orchards, resistance to strobilurins fungicides has occurred, which indicates the need to properly rotate fungicide chemistries when spraying to control this disease. Please see this UC Almond IPM page for more information. Here are some pictures to help you identify Alternaria leaf spot: Leaf symptoms of Alternaria on Carmel Another shot of Alternaria Leaf Spot on Carmel Leaf Lesion of Alternaria found on an almond leaf. Note the zonate appearance of the lesion and black specks – these are the identifying symptoms of this disease. Another photo showing the zonate appearance of the lesion.

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David Doll0 commentsAugust 20, 2011Diseases

Reports of Hull Rot

I have been getting a few calls regarding hull rot. I have covered this topic before (link to previous article), but some of the particular questions regarding the disease are answered below. What is Hull Rot?Hull rot is infection of the almond hull by two fungi, Rhizopus stolonifer (common bread mold), or Monilinia fructicola. These pathogens are common throughout the environment, and are, in this case, serving as opportunistic pathogens. Upon infection, they release toxins that are translocated into the fruiting wood, which kills the wood, causing crop loss. What do you mean by opportunistic pathogen?Once the hull splits, the perfect micro-climate for fungi is created. The hull is full of nutrients and water – the two things that fungi want from a host. Since the spores of these fungi are found throughout the air, they invade the newly split tissue, infecting, and completing their life cycle. By reducing the “home” for the fungi, we can reduce the number of hull rot strikes. These strategies include reducing the water and nutrient content of the hull. How can I prevent Hull Rot?Asking “How to prevent” is a good start. Prevention is the key to reducing hull rot strikes. Hull rot often affects high vigor orchards. Nitrogen should not be applied after kernel development is completed. This is typically the end of May, but this year it extended into mid-June. A slight to moderate water stress at the onset of hull-split should be applied. Applying both of these practices to the orchard has been shown in multiple research trials to reduce hull rot by 80-90%. Often times, this is all growers need to do to prevent severe hull rot issues. I tried those things, I still have hull rot!Severe outbreaks of hull rot have been observed. Last year, hull rot infections occurred in many orchards. I

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David Doll0 commentsAugust 12, 2011Diseases

Summer Foliar Disease Epidemics – Scab

Scab lesions found on almond twigs. I have scab all over my orchard. What can I do? Short answer – nothing. It is too late to expect control of the disease. Once the fungus infects the leaf and produces visible symptoms, that is a clear indication that the fungus has colonized the inside of the leaf. No fungicide completely penetrates the leaf surface, which makes it impossible to cure an infected leaf. Furthermore, spraying a fungicide upon seeing a disease epidemic will end up doing more harm than good. Since the fungus has completed it’s life cycle several times (Scab is a polycyclic disease), the population within the orchard is so high that making an application can lead to the selection of a fungicide resistant strain of scab. Scab lesions found on the underside of almond leaves. If every tree has scab, what can I expect? Defoliation. As the fungus colonizes the leaf, it begins producing spores which are the reproductive structures of the fungus. These spores are produced and move from an infected leaf to a healthy leaf. Upon complete colonization, the tree detects that the leaf is infected, forms an abscission layer which causes the leaf to fall from the tree. If enough leaves are infected on the tree, total defoliation can occur – and in most cases, is expected. Symptoms of scab observed on the top-side of almond leaves. What should I do? Since scab overwinters on the infected twigs in the form of lesions/fruiting bodies, population reduction is based upon controlling the disease present within the field, and coverage of susceptible tissues in the spring. Dormant applications of of copper and oil or chlorothalonil and oil have been shown to reduce summer populations of scab, and are recommended in orchards that have experienced leaf loss the previous summer/fall. Prophylactic sprays

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