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 Doll12 CommentsApril 25, 2015Diseases

Managing Phytophthora within Almond Orchards

I have been in several orchards this year which have been determined to have Phytophthora Crown Rot. This disease is aggressive and can cause rapid decline of any aged tree. Conditions that favor disease include excessive periods of saturated soils and cooler temperatures, which are common in the late winter and early spring. An infected tree can be identified by a rapid collapse of the canopy and the observation of a canker extending from the soil. Often, when soil from around the crown is removed, the canker can be observed. The occurrence of Phytophthora has becoming more common over the past few years. Reviewing my records over the past 7 years, farm visits regarding Phytophthora disease diagnosis and management were around 3 per year. Over the past two years, I have seen a 133% increase in annual visits. My notes suggest that this is due to lower quality of soils in which orchards are planted, poor selection of rootstocks for these soils, and mismatched irrigation scheduling for the soil type and tree size. In managing this problem, the industry has relied on several cultural and chemical control methods. Below are a few of the cultural practices, chemical management, and problems/concerns regarding Phytophthora root and crown rot. More information can be found at the UC IPM website. Rootstock selection. More peach-almond hybrid rootstocks are being planted in new orchards. These rootstocks are more vigorous and tolerant of toxic salts, but are susceptible to Phytophthora. In general, plum parentage rootstocks (e.g. Marianna-2624) are the most tolerant. Peach rootstocks, which include Nemguard and Lovell are moderately tolerant, while other complex hybrids (Viking and Atlas) have demonstrated some tolerance in greenhouse tests. Peach-almond hybrids (e.g. Hansen 536) are the most susceptible. Newer rootstocks remain untested (e.g. Krymsk-86, Empyrean-1, etc). In some cases, varieties are being planted on different

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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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Brent Holtz9 CommentsJanuary 17, 2012Horticulture

Plant New Trees High!

Written by Brent Holtz, UCCE San Joaquin CountyOne of the worst things that can happen to young trees from nurseries is that they are planted to deep. Sometimes  they are initially planted at the right height, but then a berm is thrown up around the trees and their crown (the graft union between the scion and rootstock) is covered with the soil from the berm. I saw many diseased trees last spring that had Phytophthora root and crown infections and every one of them had their graft union below the soil line; sometimes the union was more than six inches below. If you are planting or replanting, trees should be planted high on small mounds as shallowly as possible. Planting depth after settling should be no deeper than in the nursery, and the graft union should always be well above the soil line. Try to follow some of these planting tips: 1. Dig a hole deep enough so the roots are spread out and not cramped, 2. Plant the trees so that the nursery soil line is above the current soil line, 3. Plant the highest root a little above the soil line and then cover it with extra dirt, and 4. When planting allow for 3-6 inches of settling in the planting hole. I have never seen trees die from being planted too high, but I have seen many trees killed by being planted too low. This is especially true in heavy soils with a high clay content. These soils have slow water percolation, drain slowly, and remain saturated longer than well drained sandy soils. The mound around the tree trunk forces excess water to drain away from the tree, thus reducing the length of time the crown is exposed to excess surface moisture. Saturated soil conditions can occur

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David Doll1 CommentSeptember 28, 2009Diseases

Phytophthora Root and Crown Rot of Walnut and Almond

Root and crown rot of fruit and nut trees have caused major losses within orchards around the world. First reported in California in 1912, Phytophthora, which means “Plant Destroyer” in latin, causes root and crown rot throughout the San Joaquin and Sacramento valley. The problem tends to be more severe in areas of clay to clay-loam series soils, which can be attributed to the higher water holding capacity and poor drainage of these soils in comparison to sandier soils. The effect of Phytophthora spp. on root systems is often not noticed until the above ground parts of the tree show symptoms. Symptoms of Phytophthora damage are usually observed by poor tree vigor, loss of scaffold branches, and gumming or bleeding found around the tree trunk. Orchards affected by Phytophthora crown rot can suffer tree loss quickly, especially during the spring when winter-infected trees collapse during the first hot spell and during periods of high heat in which over-irrigation easily occurs. This is in contrast to tree loss that is attributed to root rot. In this case, by the time most growers recognize the symptoms, several trees are in a declining state due to a long period of infection (several years). Careful observance of the orchard can reveal problems and warrant remediation before the manifestation of severe orchard losses. Figure 1: Overall view of a walnut tree with Phytophthora. Note the collapse of the tree from top to bottom. As the disease infects the roots or crown, flow of water to the highest branches is compromised, with upper branch dieback being the first above ground symptom. In the orchard, symptoms of crown rot usually include cankers that are centered at the crown or lower part of the trunk. Removal of bark in affected areas will reveal brown or black sap-wood that

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