David Doll4 CommentsDecember 15, 2009Diseases

Inquiries about Bacterial Canker on Almond

This week I have had a few questions regarding post harvest treatments for Bacterial Canker of almond. I thought it would be a good idea to use these questions as the theme for this weeks post. What is bacterial canker?Bacterial canker is commonly found on Prunus species (Peach, cherry, apricot, plum, almond, etc.). Caused by Pseudomonas syringe, a common surface inhabiting bacteria, the disease is often observed in the spring. Symptoms include limb dieback with rough, irregular cankers (Figure 1). Cankers do not extend into the soil and a very unique smell is present. In some cases, cankers do not form, but a “sour sap” stage occurs. This stage is characterized by brown inner bark with a distinctive fermentive, sour smell – hence the name. Generally, almond trees in the 2nd through 5th leaf are affected. Figure 1: Almond tree killed by bacterial canker. Note that the canker has an irregular shape and does not extend into the soil. How does bacterial canker spread?Pseudomonas syringe survives on plant surfaces and is spread by splashing rain. The bacteria is found throughout the entire orchard and is impossible to eradicate. The disease commonly occurs in the spring during periods of mild temperatures and high humidity. It does not spread like other diseases, but is common on trees that are stressed. Therefore, soils that have poor water/nutrient holding capacity (i.e. sand), have been replanted with several generations of Prunus sp. orchards, and have the presence of ring nematode are prone to this disease. Figure 2: Photo of an almond with a scaffold infected with bacterial canker. Note the irregular margin of the canker.What can I do to prevent the problem?In soils that have exhibited the disease, pre-plant fumigation with Telone or Methyl Bromide (if it is still available) provides the ability for the

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David Doll3 CommentsNovember 30, 2009Diseases

Pruning Wound Cankers Found in Almond Trees

Making pruning cuts on almond trees is risky business. Every cut provides the opportunity for fungal/bacterial pathogens to enter the tree (Think: a cut on you hand can allow bacteria to enter). Once the pathogens enter the tree and successfully colonize, poor tree performance and shortened orchard life can be expected. Figure 1: Fungal canker associated with a pruning wound on a young tree. Upon bark removal, canker growth and damage is evident. Fungi are the most commonly found pathogens invading tree wounds. In figure 1, a large pruning cut was made on a super-vigorous first leaf almond. The pruning wound became infected with the fungi Eutypa which grew through the tree causing a large canker. This canker did not become noticeable until the third leaf. Upon bark removal, it is clear that the canker grows outward from the point of origin. As the canker continues to grow, it has the ability to kill branches by girdling (Figure 2), weaken scaffolds which then break (Figure 3), and/or killing the tree by girdling the trunk (Figure 4). Botryosphaeria is another fungi that may cause this problem. Figure 2: Scaffold killed by fungal canker girdling the vascular tissues of the branch. Figure 3: Scaffold split caused by weakening of crotch angle by invading fungi. Figure 4: Tree killed by wood fungal canker girdling the vascular tissue of the tree trunk. Bark removal shows the advancement of the canker. Irregardless of the possible infection by fungi, trees need to be pruned to shape (young trees), remove unwanted branches, and remove dead/diseased tissue. Knowing this, how can we prune the tree and reduce the chance of fungal infection? Most fungi require moisture/high humidity/rain event to produce spores. These spores are usually transferred by wind-blown rain. The “window of opportunity” for these fungi to infect

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David Doll3 CommentsNovember 10, 2009Diseases

Replant Disease of Almond

Written By Greg Browne, USDA-ARS UC Davis When orchards are replaced, growth and productivity of the succeeding generations of trees are often suppressed by “replant problems” unless precautions are taken. Replant problems can result from interacting physical, chemical, and biological factors, but the biological aspects usually dominate. Growers can minimize physical and chemical contributions to replant problems by pre-plant ripping and other site remediation practices and amendments to insure good soil water drainage, good soil structure, and optimal soil chemical properties (e.g. pH, soil extract electrical conductivity, etc.). Of course, appropriate soil tests are needed to evaluate the latter properties. Dealing with biological contributions to replant problems should also start with some homework—sampling for nematodes. Although plant parasitic nematodes (ring, lesion, and, on some rootstocks, root knot nematodes) probably contribute to replant problems in less than one third of California’s replanted almond and stone fruit orchards (rough estimate), it is important to sample your soil to check for them before replanting. Root damage caused by the ring nematode in sandy soils predisposes almond and other stone fruit trees to bacterial canker disease. In addition, although not common at most sites, Phytophthora species, Armillaria mellea, and Verticillium dahliae infest some orchard soils and can cause tree stunting, decline, and death in young replanted orchards as well as in established ones. It is difficult to detect and assess populations of these pathogens by soil sampling, and therefore a history of disease caused by them in the previous orchard tends to be the best predictor of future replant problems they may incite. Although it varies in severity, the most common replant problem is Prunus replant disease (PRD). In our experience, it occurs nearly universally in replanted almond and stone fruit orchards in California unless precautions are taken. Figure 1: Almond trees affected by

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David Doll2 CommentsOctober 27, 2009Diseases

Almond Tree Blow-Over Problems

Written by Bill Krueger, UCCE Glenn County Periodically fall winter or spring storms with associated high winds and saturated soil conditions result in high tree losses. As an example, a March 1995 a storm brought 6 inches of rain and wind gusts up to 100 mph to the Sacramento Valley and resulted in an estimated loss of 15,000 acres of almonds worth $210 million. More recently a storm in January of 2008 with wind gusts in up to 70 mph resulted in tree losses of greater than 30% in some orchards. When trees fall over their failure can usually be classified as windthrows or wood decay related. Windthrow is when trees with sound healthy roots are uprooted as a result of strong winds and wet soil conditions. Wood decay related failures are the result of infection from wood decay fungi, which consume the lignin in the cell walls of the heartwood. Eventually these trees fall, often in wind storms. Wood decay fungi are secondary pathogens and are not capable of penetrating intact plant membranes and must rely on some type of injury to gain access. Survey work conducted by Joe Connell, Butte County Farm Advisor and Jim Adaskaveg, UC Plant Pathologist following the March 1995 storm revealed the following. Generally, as trees aged tree losses increased. Windthrow was an important factor in young orchards (Fig.1). As orchards matured wood decay became more dominant. Wood decay accounted for approximately 90 % of the tree loss in mature orchards. Of 394 downed trees evaluated: 77% had wood decay and 81% had crown gall. Crown galls function as sites of entry for wood decay fungi.Generally almonds on Marianna 2624 suffered lower losses than peach rooted trees. The exceptions included orchards in flooded areas with softened soil and unpruned orchards with dense canopies. Generally

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David Doll2 CommentsOctober 19, 2009Diseases

Sampling for Plant Parasitic Nematodes

Many growers are facing the question of replanting orchard blocks. This aspect of orchard maintenance is critical and essential in maintaining an orchard business. Outside of the general tree removal, soil and ground preparation, there are 3 key biological replant problems that must be taken in consideration when replanting. These include plant parasitic nematodes, aggressive soil pathogens and pests, and replant disease of almond. This blog will focus on the first mentioned aspect of replant problems: nematode sampling and nematode control. In almond production, there are three main parasitic nematodes that are of concern – Ring, Rootknot, and Lesion. Depending upon your soil type, planted rootstock, and previous cropping history, the populations of these nematodes can vary within the soil. It is important to keep in mind that plant parasitic nematode populations are directly related to tree production – in other words, the higher the number present, the more tree damage to expect. A previous blog describes the affect of nematodes on tree growth. Orchard Sampling to Determine Nematode Populations. To determine nematode populations, soil samples collected from the root zone area of a tree should be sent off to a laboratory for processing. In collecting samples, the following procedure should be used: 1. By using a soil map, identify soil texture changes within the orchard block, 2. Time the sampling to occur after the year of cover cropping or fallow period – if one occurred, 3. Attempt to sample from different soil textures and areas of previous tree performance. A good sampling strategy for a 40 acre block can be seen in figure 1. Please note the sampling in different soil textures and random samples throughout the field, 4. Use a soil tube and sample the 4″-16″ depth of the profile – this is the area of most active

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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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David Doll0 commentsAugust 24, 2009Diseases

Nematodes and Tree Growth

A grower made a request for a visit to his orchard due to poor growth by his replanted first leaf trees. The trees were replacing trees that were lost to heavy winds from the previous summer. Within the orchard, there were several different pollinators, and all of the trees were planted on Lovell rootstock. This orchard was located in a sand/loamy sand soil series, irrigated with solid-set sprinklers, with trees ranging from 1rst-15th leaf throughout the block. Often with replanted trees in an established orchard, it is very difficult to obtain vigorous growth. The trees usually are partially shaded out, receive either too much or too little water and fertilizer, and generally struggle with soil problems involved with replanting. The replants throughout the orchard were showing signs of low vigor, which include poor shoot growth, thinned canopy, and a general weak appearance (Pictures 1 and 2). The trees showed no other signs of disease on the scaffolds or trunk. Upon root excavation, a poorly developed root system was observed. There were no signs of fungal root infection or any other aggressive pathogens, vertebrate pests, or insects. Picture 1: A first leaf almond tree severely stunted by high nematode populations. Picture 2: A first leaf almond tree moderately stunted by high nematode populations. At this point, I decided to sample for plant parasitic nematodes. Sampling for nematodes is relatively easy: Remove the top 6″ of soil from the sampling locations, and use a soil sampling tube* to remove soil from the depth of 6″-18″ and place the soil in a bucket. Do this from 4-5 locations and mix the soil thoroughly within the bucket. Pull out 2-3 pounds of soil, place it in a labeled plastic bag, and place in a cooler or refrigerator until the sample can be sent to

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David Doll3 CommentsAugust 17, 2009Diseases

Armillaria Root Rot (Aka Oak Root Fungus) of Almond

A call last week revealed a disease issue in which there are very few solutions. A grower called with concerns of several trees recently collapsing from the heat. The decline has been relatively gradual until a week or go, in which the trees suddenly turned brown and are now dead (Picture 1). This problem has been ongoing for several years in which he would replant the trees after a tree site fumigation. The orchard was a 10th leaf Nonpareil, Fritz, and Aldrich orchard, planted on Nemaguard. Picture 1: Overview of an almond tree affected by a root disease. The orchard was in a clay-loam soil series, flood irrigated with district water once every 10-13 days, and appeared to be in good shape outside of the affected areas. Affected trees were usually surrounded by younger, replanted trees confirming the replanting that the grower has done in the area (Picture 2). Furthermore, the affected trees appeared to be in groups, and this was confirmed by a Google Earth image once I returned back to the office (Picture 3). All this suggests a biotic agent causing disease, and seeing a total collapse of the tree, I began to investigate the root system. Picture 2: “Hotspot” of area infected by Armillaria root rot. Picture 3: Google Earth Aerial Image of the orchard affected by Armillaria Root Rot. Please note the large holes within the orchard canopy indicating missing or dead trees. Upon root excavation, the tell-tale signs of Oak Root Fungus or Armillaria Root Rot became relevant. Hitting a surface root and scraping back the bark, the white mycelial fans became evident (Picture 4). Furthermore, the roots possessed a smell similar to what we would expect from mushrooms – similar to the smell of mushrooms in the grocery store. Further bark removal and scraping

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Brent Holtz10 CommentsJune 30, 2009Diseases

Hull Split Part 2: Hull Rot of Almond

As almond trees approach harvest, at about mid hull split, clusters of dry leaves begin to appear scattered through the tree canopy. Individual spurs, small shoots or entire small branches may collapse due to hull rot infections (Photo 1). The loss of fruiting wood, especially in the lower parts of the tree, can negatively affect yield for years to come. Nonpareil is usually the most severely affected cultivar though Sonora and Kapareil can also sustain extensive damage. Hull rot is caused by either of two fungi, Monilinia fructicola or Rhizopus stolonifer. Monilinia fructicola is best known as one of the brown rot fungi and R. stolonifer is often called the bread mold fungus, and will turn bread left out black and moldy. Photograph 1: Hullrot of almond caused by Rhizopus stolonifer. In the southern San Joaquin Valley I believe that Rhizopus is the primary pathogen responsible for hull rot while Monilinia may be more important in the Sacramento Valley. These two organisms are very different but can cause similar disease symptoms on almond. As the name implies, a lesion or dryish rotted area develops on the hull, and dense masses of Rhizopus spores produce a powdery dark gray to black growth between the hull and the shell (Photo 2). Monilinia spores are buff-colored and can be seen on inner and outer hull surfaces. The nut meat is not damaged, but a toxin produced in the infected hull moves from the hull into the neighboring leaves and shoots causing death of these tissues. Neither Monilinia nor Rhizopus are able to invade the healthy outer hull surface. Only after hull split begins can spores gain access to the inside of the hull and initiate infections. Once hull split starts, trees are at risk of becoming infected. One or both pathogens may be

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David Doll8 CommentsJune 16, 2009Diseases

Foamy Canker in Almond

This is one of the most visually moving diseases found in almond. Once upon the location of the problem, not only is the disease obvious by looking at the trees, but the air is also filled with a fermented-like smell reminding one a bar when entering for happy hour. Overall Orchard Information: The orchard I was called out to visit was described as a complete loss to foamy canker. To me this came as a bit of a surprise as foamy canker typically comes later in the season. By time I have arrived, the grower was in the process of removing the orchard. The entire orchard was exhibiting symptoms of foamy canker (Picture 1), poor growth and dieing trees. Photo 1: Overview of the almond variety Fritz with foamy canker The orchard was a Nonpareil-Fritz-Monterrey on Lovell planted on clay/clay loam soil. The trees were in their 4th Leaf trees with average growth. Nut set of the trees was decent, but not great. Speaking with the grower, he mentioned that the orchard has a high water table, found at about 7 feet with subbing up to 4-5 feet. Quality of the water was unknown. The site is also high in boron within its soil, and has a history of verticillium wilt (old tomato and melon ground). Overall, the discussion yielded that there were many issues from the start with the orchard – over 30% of the Fritz variety planted in the first year did not survive and needed to be replanted. Irrigation of the trees tends to be minimal due to the high water table. The grower does not irrigate often – only 8 inches last year through double line drip. Nitrogen usage is minimum, but tree vigor is high due to being the first generation orchard on old row

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