David Doll0 commentsApril 17, 2020Diseases

Shot Hole Fungus on Almond

Shot Hole is a fungal disease caused by Wilsonomyces carpophilus. The disease is able to infect leaves, fruits, and green wood. Leaf infections are characterized by a lesion that is between 1/8″-1/4″ (3-6 mm) in diameter, with a definitive yellow halo. Often, the lesion has a small black fruiting body in the center. As temperatures warm, the lesion drops from the leaf, leaving a hole. Fruit infections are generally smaller in diameter (~1/8″), appearing purplish-brown, slightly cork-like in appearance, and are raised. Severe fruit infections can kill the developing nut or cause deformities, impacting quality. Twig infections are similar to fruit infections. In severe cases, multiple lesions may girdle the green branch, causing dieback. This disease is very common in almond producing areas around the world. It survives/overwinters on infected twigs and as spores within leaf buds. Infection occurs when there is ample moisture and temperatures above 36F/2C. In warmer conditions, the fungus can produce spores and infect leaf tissues in less than 6 hours. Multiple infection cycles can occur within a season due to re-occurring rain events, which can cause severe defoliation. Due to the requirement for leaf wetness, this disease tends to be more prevalent in production areas that have significant rainfall after leaf-out. Frequent periods of leaf wetness that are greater than 6 hours will increase the risk of disease. In order to prevent infections, fungicides should be applied prior to rain events. These products kill the spores as they begin to germinate, reducing the number of infections. If rainy periods persist, multiple fungicide sprays will be needed. For more information on control, please see the Univ. of California Integrated Pest Management website. It highlights the life-cycle and fungicide chemistries to utilize for control. When diagnosing, be aware that there are a lot of problems that look

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David Doll8 CommentsSeptember 29, 2019Diseases

Phytophthora: An Update

As previously written (2009 article and 2015 article), Phytophthora is a plant pathogen of almond trees. This fungal-like water mold is common throughout the various almond growing regions of the world, with 6* or more known species causing disease. When conditions are conducive for infection, the pathogen directly invades plant tissues at the crown or roots, leading to loss of nutrient and water uptake, causing tree decline and eventual death. The rate of infection and tree loss depends on the inoculum present, the choice of rootstock, environmental conditions, and cultural practices that may favor disease. Phytophthora inoculum can be endemic to the orchard area or introduced through a variety of ways. Low levels of Phytophthora probably exist in most soils, but typically are not at the levels or location to cause infection unless there are prolonged conditions favoring disease.  Generally, introduction of inoculum into the area of close proximity to the plant is most likely responsible for infections and tree death. This includes the crown or base of the tree where the pathogen infects the plant just below the surface of the soil. These types of infections are most common and can lead to rapid tree loss.  Root infections, which occur when soils are saturated with water, lead to a gradual decline. Finally, aerial Phytophthora happens when spores are deposited on limbs or in crotches of the trees through dust or flooding, followed by wet conditions (e.g. rain). Surface water is believed to be the major contributor of Phytophthora inoculum in perennial nut orchards. Surveys have found several species of pathogenic Phytophthora in high elevation, upstream waterways of the major rivers in California. Due to the ability of the pathogen to survive in water, this pathogen is probably found in all surface water used to irrigate trees worldwide.  This inoculum

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David Doll0 commentsAugust 8, 2019Diseases, Field Notes

Field Note – Powdery Mildew of Almond

Written by Cameron Zuber, UCCE Merced County Some interesting almonds came into the office and, no, this is not a setup for a joke. In the middle of July, some almond fruit nuts from Le Grand, CA were brought into the Merced County UC Cooperative Extension office. There were various signs of possible pest damage and disease infection. These included black growth inside the hull indicating possible Rhizopus stolonifera fungus which may lead to hull rot; grayish-black spots indicating scab (Cladosporium carpophilum); and gumming which may indicate bacterial spot (Xanthomonas arboricola pv. Pruni), anthracnose (Colletotrichum gloeosporioides), or bug feeding. However, the greatest amount of damage were odd areas of russeting and cracking on the hull. After speaking with person who brought in the samples, there was not any indication of leaf or shoot die back near the nuts which starts to rule out hull rot and lab tests did not show any fungi that would cause hull rot. Lab test also showed some instances of almond scab growths, but none of these pest or disease symptoms were at a level to be extremely concerned about. Also, none have been shown to cause the odd cracking on the hull, so what was the cause, should you be worried about it, and how do you fix it? The cause was found through some digging and a lot of help from farm advisors and plant pathologists. It was determined that the cause was most likely a false powdery mildew (or powdery mildew-like) fungus of the Acremonium species. Normally these organisms live on dead or decaying organic matter (i.e., saprophytes), but do occasionally infect plant material and, for almonds, result in the russeting and cracking seen in the above fruit nuts. It is called “false” or “like” powdery mildew because it shows similar symptoms

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David Doll1 CommentJuly 25, 2015Diseases

2015: Lower Limb Dieback Returns

I have received several calls and comments regarding the severity of lower limb dieback (LLDB) in almonds this year. This topic has been discussed to a greater extent here, but I thought I would highlight a few points and some updates to keep in mind if you are observing these symptoms in your orchard. LLDB is used to describe branch death that occurs in the lower half of the canopy. The limbs tend to start the year off normally, but at some point – usually after a hot spell- the leaves on the affected limbs begin to yellow, and eventually the whole branch collapses. Observation of symptoms usually occurs between late May through late July, with symptoms delayed in years with mild springs. LLDB is very common on the ‘Padre’ variety. So what causes LLDB? The “jury” is still out, but there are a few hypothesis. The current thought is that over-irrigation in the spring followed by under-irrigation during the summer months can worsen or cause LLDB. Over-application of water often occurs in the spring time due to the variable weather conditions (e.g. unexpected cool spells and rain). When it cools off, it is easy to over-apply water especially if the irrigation scheduling is based on the 30 year average. A good example was this past May, in which we experienced an estimated 5.68″ of almond water use (ETc) and had rainfall events that dropped as much as 0.25-1.0 inches of rain in Merced. The 30 year average estimates that we would need to apply 6.44″ of water to match ETc, and if followed, over-irrigation with an inch or more water may have occurred. Furthermore, many orchard operators tend to begin too early with the first fertigation, often beginning before transpiration levels of the tree draw down soil moisture, which keeps the soil

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David Doll3 CommentsJanuary 1, 2015Diseases

Bacterial Spot of Almond in California: Update on the Disease and Management

written by Dr. Jim Adaskaveg (UC Riverside), Brent Holtz (UCCE San Joaquin), Roger Duncan (UCCE Stanislaus), and David Doll (UCCE Merced).   In the spring of 2013, we reported a high incidence of bacterial spot in some Sacramento and northern San Joaquin Valley almond orchards, especially on the ‘Fritz’ variety.  It was also found on ‘Nonpareil’, ‘Butte’, ‘Carmel’, ‘Monterey,’ and ‘Price’ but at much lower levels.  The disease has not been found in orchards in the southern San Joaquin valley. Bacterial spot is caused by Xanthomonas arboricola pv. pruni and has also been referred to as bacteriosis, bacterial leaf spot, or bacterial shot hole. Bacterial spot occurs on leaves, twigs, and fruit of almost all Prunus spp.  The pathogen commonly causes bacterial spot of peach in the eastern United States and is one of the major foliar diseases in high rainfall years. On almonds, it is a significant problem in Australia because of mid-season rain events.  In 2014, we verified the presence of the disease at previous and additional locations in California, although at levels much lower than in 2013.

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David Doll0 commentsApril 11, 2014Diseases

Jacket Rot on Almond

I have been receiving a few calls about issues with deformed almonds, particularly with the variety of Monterey. When looking at the nuts, it appears that the damage is associated with the jacket. This appears to be jacket rot. Samples submitted were placed in a humidity chamber and produced the gray/brown “fuzz” that is common to Botrytis, one of the causal agents of jacket rot. Based on the weather, this appears to be an issue with the four day rain event that occurred as the jackets were splitting on the later blooming varieties. I dont suspect a major problem, unless we re-enter a period of rainy, cool weather.

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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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David Doll2 CommentsApril 25, 2011Diseases, Weeds

Herbicide Drift or Fungal Disease?

Figure 1: What caused this damage: Herbicide drift or fungal disease? Leaf samples often appear on my desk with notes asking to identify the problem. More often than not, symptoms appear shothole-ish in nature, and growers are questioning their fungicide program (Figure 1). These symptoms, however, also appear from drifting herbicides. How can one tell the difference between three common herbicides and the fungal disease shot hole? Answer: look for fungal fruiting bodies. Shot hole infected leaves can be determined by the black fruiting body that is centrally located in the lesion on the top side of the leaf (Figure 2). Figure 2: Shot Hole affectedalmond leaf. Note the blackfruiting body in the centerof the lesions.  What about telling the difference between the three herbicides? This one is a little bit harder, but it comes down to the coloring of the leaf tissue surrounding the lesion. Kurt Hembree, Weed Management Farm Advisor in Fresno County and Brad Hanson, UC Weed Specialist, have noted the following:  lesions caused by paraquat are localized, tan in color, and remain attached (figure 3). Carfentrazone (Shark) lesions have halos that lack color (figure 4). Oxfluorfen (Goal) lesions tend to have a defined halo, sometimes purplish and/or yellow in color (figure 5).  Both of these herbicides will cause lesions to fall from the leaf. Figure 3: Shot Hole like damage caused by  Gramoxone (Paraquat). Note that the lesions do not fall from the leaf To help with the diagnosis, observe the location of the damage. Is it over the whole tree, or just lower few branches? Is it just on the outer rows? Is it across both varieties? What is the history of herbicide use? Herbicide damage tends to be found on the lower portion of the tree, with damage on all varieties within the orchard. Drift damage also tends to

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