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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Almond Shaker Damage

Shaker damage is commonly observed across orchard operations. This damage occurs when the force of the shaker is greater than the strength of the bark, causing it to tear away from the tree. The obvious damage not only impacts tree vigor, but also provides an opportunity for infection by wood canker fungi which can kill the tree. If the shake is properly timed and executed, shaker damage can be greatly reduced (and even eliminated) within an orchard. A common misconception is that trees that are kept too wet during the harvest period will be damaged by the harvesting process. Interestingly, this has never been supported in research nor found to be consistently true across operations. Research in the mid 90’s by Gurusinghe and Shackel found that withholding irrigation during July through harvest did not reduce bark damage of the trees. To further elaborate, they found no difference in bark strength with respect to shaker damage for almond trees grown under various irrigation treatments. Irrigation treatments included a wet treatment and dry treatment, in which mid-day stem water potential (SWP) was maintained at -9 bar and -20 bar, respectively, through the month of July/August. Despite this range in tree water status, both treatments exhibited the same gradual increase in bark strength through July and August. Based on this, the researchers concluded that water stress did not influence the timing or rate of tree bark strengthening. Based on the lack of direct effect of moisture status on tree bark strength, it is thought that the damage observed in wet areas of the orchards is most likely due to the delays in ripening. Research has found that decreased water stress (Goldhamer and colleagues, 2006) and high nitrogen status (Saa and colleagues, 2016) both  delay ripening. These trees, when shaken at the same time

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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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Timing Fungicide Sprays: Thoughts for a Wet Spring

Improper timing of fungicides wastes money and leads to crop loss from disease. Over the years, I have seen several disease epidemics that were due to the improper timing of a fungicide spray. These were observed in both wet and dry years, indicating the difficulty and general misunderstanding of disease and spray application timing. Proper timing of fungicide applications is difficult. Operational constraints in applying fungicides leads to mis-timed (too early, too late) or poor applications (every other row, applying too fast). This is often a function of equipment availability, whether by grower/operation or custom applied spray. Add to this aspect the various layers of fungicide availability and delivery timing, the changing pathogens and disease conditions, as well as the general misconceptions of various diseases, it is very easy for problems to occur. In order to control diseases, as advocated in earlier postings, it is best to design a fungicide plan prior to the start of the season. This provides the opportunity to review reference materials and determine which fungicides are best for differing stages of the crop. This plan also creates the opportunity to provide options for differing modes of actions at each timing to help with pricing, and reduces the burden of having to put the plan together when the season has started (and there are several other demands). Personally, I like to design the plan for a wet year and remove fungicide timings if dry conditions occur. An example of a plan with different modes of actions can be found here. Diseases don’t just occur. They require a susceptible host, the presence of a pathogen and suitable environmental conditions. For most almond foliar or bloom diseases, the pathogen is throughout the environment and most of our varieties are susceptible to one disease or another. The largest variable

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Pest Alert: Peach root-knot nematode found in Merced and Kern almond orchards in California

Written and provided by UC IPM. Link to the original article (click here). In brief: Peach root-knot nematode is newly discovered in California, but its current distribution is unknown. Peach root-knot nematode is capable of infecting most Prunus rootstocks currently used in almond and stone fruit production. Rootstocks resistant to previously detected root-knot nematodes may not be resistant to peach root-knot nematode. The California Department of Food and Agriculture rated this nematode as an A quarantine-actionable pest. Contact your local county agricultural commissioner’s office if you suspect you have peach root-knot nematode (uneven and poor tree growth, stunting, and root galls on resistant rootstocks). The peach root-knot nematode (Meloidogyne floridensis) was recently discovered in California and has the potential to infect many of California’s economically important crops. At the time of this writing, it is not clear how wide-spread this nematode is in California. The California Department of Food and Agriculture (CDFA) rated this nematode as an A quarantine-actionable pest. Since the early 1960s, the rootstock Nemaguard and others such as Marianna 2624 and Myrobalan 29C, have protected Prunus crops (almond and stone fruits) from attack by southern root-knot nematodes (M. arenaria, M. incognita, and M. javanica), which are common in California. In contrast, peach root-knot nematode is capable of infecting Nemaguard and peach-almond hybrids. Research in Florida has identified some differences in response among Prunus rootstocks, but peach root-knot nematode itself has variability that allows some populations of it to infect peach and peach-almond hybrids rootstocks that are currently used in California. A consortium of University of California (UC) Cooperative Extension, UC Riverside and CDFA is working with the county agricultural commissioners of the affected counties, the growers, respective crop consultants, and the Almond Board of California to mitigate the potential impacts of peach root-knot nematode. Emphasis will be

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Alkaline treatments have been shown to reduce hull rot

Recent research has found that alkalizing treatments applied at hull split are able to reduce the severity of hull rot caused by Rhizopus stolonifer. Over the past two years, work by Dr. Jim Adaskaveg and colleagues out of UC Riverside has repeatedly shown that several products reduce hull rot incidence. These products included dipotassium phosphate (applied as the product diKaP) applied at 48 ozs/acre, calcium hydroxide applied at 320 ozs/acre, and Cinetis applied at 24 fl. ozs/acre. A single application of each of these products made at 5% hullsplit was shown to be as effective as several fungicide combinations and reduced hull rot strikes by over 75% in comparison to the untreated control. Multiple applications, with one made at early suture split (traditional first navel orangeworm (NOW) timings), did not seem to reduce strikes further. The cause of the reduction in strikes is unknown. It doesnt appear to have a fungicidal effect on the fungus as often the fungus is present within the hulls. The current thought is that these products neutralize the fumaric acid that is produced by this fungus (this is the acid responsible for limb death).  It could also be due to an increase in tolerance to the toxin from the foliar product. Work is ongoing. With these products, timing is key. Apply around 5-10% hull-split. Since these products may be tank mixed with NOW products as well as other fungicides, confirm compatibility by running a jar test. If struggling in managing hull rot, consider trying these products on a selected blocks to see if they provide any reduction in observed damage. Just remember to leave an area untreated in order to determine treatment effectiveness.  Finally, keep in mind that the use of these type of products for hull-rot enters the “grey world” of registration.  Follow appropriate

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2018 Almond Bloom Considerations

2018 almond bloom is around the corner. This year will probably be one of the earliest almond blooms we have had in Merced County. This accelerated bloom is due to the warmer, sunny weather experienced through January. Depending on conditions following flowering, hull-split and harvest should also be earlier. Weather for the coming bloom period looks to be favorable for the San Joaquin Valley. Warm conditions (temperatures in the 70’s) and no rain in the forecast provides the conditions that are conducive for rapid flower development. Thankfully, these conditions will not favor disease. Early bloom sprays may not be needed for many tolerant varieties (e.g. Nonpareil), unless heavy dew and susceptible varieties (i.e. Butte) are present. Regardless of weather, a fungicide spray made at full bloom/petal fall should be considered. This fungicide should provide protection for brown rot, shot-hole and jacket rot. Of these three, jacket rot is one of the harder fungi to control with the modern fungicide chemistries as the strobilurins (FRAC 11) and DMIs (FRAC 3) are not effective. This spray timing will provide protection for any rain events that would occur within two weeks of application. Successive in-season sprays may be needed if the weather turns wet. Further information reagrding fungicide timing and efficacy can be found here: http://ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf . Fungicide applications should be made at a time of day to reduce bee exposure. Applications later in the day will reduce exposure as bee foraging is decreased in the afternoon. If applying at night, allow enough time for the fungicide dry before bee foraging begins the next day. Also, avoid any addition of surfactants with the fungicide unless directed by the fungicide label. Many more management practices for bees can be found at The Almond Board of California’s website: http://www.almonds.com/pollination . The first irrigation will be tricky

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New pesticide labels available to help control pruning wound infections

Topsin® M WSP and Rally® 40WSP now have 2EE labels to protect pruning wounds on almonds (and other stone fruit trees and/or grapes) from canker disease infection. These products should be especially helpful in protecting young trees where disease infection of relatively large pruning cuts may cause extensive damage.   —-Canker diseases listed on the Topsin®M and Rally® labels—-   Topsin®M WSP Rally® 40WSP Eutypa Eutypa Cytospora Cytospora Calosphaeria pulchella Calosphaeria pulchella Lasiodiplodia theobromae Lasiodiplodia theobromae Botryosphaeria Botryosphaeria* Phomopsis Phomopsis* *suppression, only   The labels are very similar in many regards.  The following are recommended on both labels: Application within 24 hours of pruning is highly recommended. Assure complete coverage of all cut surfaces. Add a registered spray dye to better assess coverage. Additional application after about 2 weeks, especially if rain, irrigation, or high humidity occurs. Product combination (tank mixing) for best results and resistance management (Topsin M is a FRAC 1 fungicide, Rally a FRAC 3, and both have single site – high resistance risk – activity). Use of an organosilicone surfactant to improve pesticide penetration into pruning wound surfaces. (It is up to the grower to select and use a crop-safe organo-silicone with these products on pruning wounds).   There are some differences in the labels that limit how they can be used separately or combined. While the labels recommend tank mixing for best results and resistance management, consider the following: The Rally label does not allow painting the product on pruning wounds, while the Topsin M label does allow painting In addition, at the labeled rate (1.5 lb/acre), Topsin M is limited to 2 applications/year (max of 3 lbs/acre/year), while Rally is permitted a max of 3 applications/season (max of 1.5 lb/season). Therefore, if the two products are combined, painting is out and only 2 applications

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Aspergillus niger induced hull rot on almond

Written By Brent Holtz, Ph.D., UCCE Farm Advisor San Joaquin County   In September I visited an almond orchard in San Joaquin County with the Supareil variety showing symptoms that looked very similar to hull rot typically observed on Nonpareil.  Symptoms showed dieback of vigorous young shoots and lower limbs (photo 1-blighted shoots).    Leaves were stuck on many shoots and typically blighted while hulls were unusually shriveled (photo 2-shriveled hulls), with fungal sporulation observed on the inner hull next to the shell (photo 3-fungal sporulation).  The fungal sporulation, however, didn’t look similar to the sporulation typically observed on hulls infected with Rhizopus stolonifer or Monilina fructicola, the typical pathogens associated with hull rot of almond.  Many of the larger blighted shoots seemed to still be alive even though they had defoliated, while other blighted shoots were starting to push new leaves in September—not typical of blighted shoots infected with Rhizopus or Monilinia. Samples of infected hulls were sent to Dr. Themis Michailides, a plant pathologist with UC Davis / Kearney Research and Extension Center, where he isolated Aspergillus niger from 88% of the samples collected.  The other 12% of the diseased nuts collected were found to be infected with Rhizopus stolonifer, the typical bread mold pathogen commonly associated with hull rot.  Aspergillus niger has not commonly been associated with hull rot in the San Joaquin Valley, but Dr. Mohammad Yaghmour, Farm Advisor in Kern County, has also observed similar hull rot. Hull rot can reduce yields of vigorous young almond orchards in the central and southern San Joaquin Valley.  Symptoms often observed are dieback in lower limbs that often exhibit less bloom in the spring.  I’m wondering how the Supareil return bloom will be in this particular orchard next spring?  I believe severe hull rot can enhance the shading out

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Almonds Exuding Clear Gum

Several calls about gummy nuts were received this week. Of concern are nuts that are oozing clear gum along the suture or side of the hull. There are many potential causes of clear gumming, which include bug damage, boron deficiency, and internal causes from an expanding kernel. Bug damage. Most people are aware of leaf footed plant bug damage. Feeding by this insect can kill the almond embryo when the insect inserts its needle-like feeding tube into the nut. This often leaves a small pin-prick hole which results in clear gum being exuded 3-10 days after feeding. Although the risk of the bug killing the embryo decreases after shell hardening, there can be some staining of the kernel. As the season progresses, feeding by plant bugs decrease, while stink bug feeding can increase. These insects do not have long enough mouth parts to feed on the kernel, but feed on the fruit instead. They often have several feeding points on a single nut. All of these may gum. Severe feeding by stink bugs may impact kernel quality, but the economic threshold of these insects is unknown. Boron deficiency. Too little boron can cause gumming. Usually a clear gum exudes out of the side of the nut or suture line. When cutting the nut open, the kernel is often discolored with copious amounts of gum. These nuts often drop from the tree, and if not, the gum can harden and misshape the kernels, impacting kernel quality at harvest. Another sign of deficiency is a low crop set. Tree boron status should be determined by hull analysis – which has been discussed previously.  This type of deficiency is common in areas that use clean surface water and have not applied any boron to the soil. Physiological- expanding kernels. Occasionally, almonds can gum for no apparent reason.

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