Since the first reports of a new almond pest – the carpophilus beetle (Carpophilus truncatus) – came in during fall 2023, it has become clear that the beetle is widely dispersed across the San Joaquin Valley.
“My lab has identified infestations from every county in the San Joaquin Valley; we have found infestations in both almonds and pistachios, and we will likely find infestations in walnuts this fall,” said Houston Wilson, a University of California Cooperative Extension entomology specialist at UC Riverside. The California Department of Food and Agriculture has confirmed the beetle’s presence in Stanislaus, Merced, Madera and Kings counties.
Historically a major threat to almond production in Australia, the beetle – as larvae and adults – feeds directly on the nut kernel. In California, some almond growers have lost 10 to 15% of their yield – a “significant economic loss,” according to Jhalendra Rijal, University of California integrated pest management (IPM) advisor for the region. Given the prominence of almonds as a commodity, even a 1% overall reduction statewide represents an approximately $70 million loss.
“This year there has been a lot more reports from PCAs [pest control advisers]; they’re sending me the pictures of the damage and beetles,” said Rijal, noting that the increase is likely due to greater awareness of the pest.
To help almond growers identify the carpophilus beetle and develop management plans, Rijal, Wilson and their IPM colleagues have put together a visual ID guide for the beetle and the damage it causes, as well as telltale signs of navel orangeworm (Amyelois transitella) and ant damage. In particular, the experts would like PCAs and growers to differentiate between the carpophilus beetle and navel orangeworm, another key pest in almonds.
“Even though their way of causing damage looks more or less similar, we’re dealing with two different types of insects,” Rijal explained. “One is a Lepidoptera moth [navel orangeworm], and the other one is a beetle – many of the management practices and biological controls would be different for these two different things.”
TOP: Both carpophilus beetle adults and larvae feed on the kernel and cause damage characterized by fine powdery frass and nutmeat, a white-creamy color with some webbing. Often, large numbers (more than 10) of adult and larvae are found per nut. BOTTOM: Navel orangeworm larvae cause damage characterized by thicker frass and silky webbing entangled with a darker, brownish appearance. Only larvae are present at harvest, usually 1 to 3 larvae per nut. Photos by UCCE Stanislaus IPM team
To control carpophilus beetle, ‘sanitize, sanitize, sanitize’
One crucial cultural practice for managing both pests, however, is destroying the remnant “mummy” nuts – the nuts that remain in the orchard postharvest. They serve as overwintering habitat for the carpophilus beetle, as well as its sustenance for the next generation of beetles in spring.
“The best way to manage this pest is to do the orchard hygiene – continuing the winter sanitation, destroying the nuts that are on the ground and on the tree and on the berms,” Rijal said.
Based on observations in Australia and locally, carpophilus beetles tend to rely more on mummies on the ground, whereas navel orangeworm generally favors mummies in the tree canopy. Correctly identifying the pest – with help from the new ID guide – enables growers to better target and prioritize their management efforts, Rijal said.
“What we are strongly emphasizing is that growers need to sanitize, sanitize, sanitize to control both pests,” Wilson added.
Correct identification of the pest would also prevent unnecessary application of insecticides, as those used for controlling Lepidoptera such as navel orangeworm would be largely ineffective on the beetle.
Indeed, another insight shared by Australian experts is that the carpophilus beetle cannot be controlled just by insecticide.
“Insecticides are not very efficient, given the cryptic nature of these beetles; exposing these beetles to the insecticide is very hard,” said Rijal, noting that the beetle spends most of its life cycle protected inside the nut.
Mature larva of navel orangeworm (NOW) is 3 to 4 times larger than carpophilus beetle (CB) mature larva. Photo by Jhalendra Rijal
This harvest season, Rijal advises almond growers to harvest as efficiently as possible, to minimize the number of mummies that need to be cleaned up. And because signs of damage (like damaged hulls and frass) are most obvious during harvest time, Rijal said growers should review the new guide, using the photos and other resources to help identify potential pests.
Scientists are looking to expand their knowledge about this relatively new pest to California. In the coming weeks, for example, researchers are planning to survey for the carpophilus beetle in the Sacramento Valley.
“Technically it has not been found there, but we suspect that we’ll find it this fall when we go looking for it,” Wilson said.
Researchers are also collecting samples from infested orchards to better understand the biology of the species, as well as how it progresses through and responds to seasonal and climactic changes. In addition, they are analyzing data from a trial study of an insecticide that might be used as a supplemental control measure.
“This is our first full season dealing with this insect, and there are still many things we need to understand,” Rijal said. “We are continuing our research efforts on all fronts.” — By Michael Hsu, UCANR
As brown marmorated stink bug becomes a bigger problem in the Pacific Northwest, hazelnut growers are searching for better tools to manage this invasive pest. Matthew Malcolm met with Edwin Harris from the Vaughn Walton Lab at Oregon State University, who is evaluating a promising new monitoring trap to help growers combat this pest. Watch this brief interview and read more about it in Pacific Nut Producer Magazine. Don’t currently receive the magazine? Subscribe for free at: https://malcolmmedia.com/subscribe/
By the time you will read this article, pistachio nuts have almost completed 80% of the final size and their ovary is still small getting ready to start increasing its size to completion by the end of July. June is considered still too early to start seen early-split (ES) nuts. These will start showing in the second part of July and will continue developing even close to harvest.
June is the month when management of Alternaria late blight disease typically begins. However, before getting into the management of the disease let us understand, how, when, and why this disease develops in a pistachio orchard. There is a span of 2 months window (June and July) for the critical sprays that can control Alternaria late blight disease of pistachio. Alternaria late blight, as the name indicates, is caused by Alternaria species, the most common being Alternaria alternata. This species is ubiquitous in nature and can grow as pathogen in live tissues (leaves and fruit) and as secondary in orchard debris. In pistachio, the disease causes necrotic lesions in the surface of leaves where the pathogen produces abundant windborne spores under high humidity conditions in the orchard. A diagnostic feature of the Alternaria leaf lesions is the blackening of fingers after rubbing the sporulating lesions. Fruit also can be infected by Alternaria, especially those of late, second, or third harvests. In addition, if a rain occurs in late season, more disease will develop. That was the case during the tropical storm Hilary that occurred in the first half of August, resulting in severe outbreak of Alternaria late blight in pistachio orchards, even in areas historically characterized as “disease-free” under normal weather conditions in California (i.e., Westside). The fruit infection (decay) was so severe (Figure 1) that a grower admitted or reported? a 25% loss of his crop, although he had harvested twice this orchard. In addition to Alternaria blight, in the Terra Bella area where some orchards used to have some Botryosphaeria panicle and shoot blight, the disease developed in very high levels after the storm.
Figure 1. Main Alternaria late blight symptoms on pistachio: rot lesions on fruit (following the August 2023 Tropical Storm Hilary), necrotic lesions on leaves , and a closeup of large and small black Alternaria lesions.
A Brief Summary on Disease Development
Although less frequent, Alternaria spp., like Botryosphaeria spp. can cause latent infection in the spring when rains wet the fruit and leaves and spores germinate to establish infections without any expression of symptoms. As leaves mature and start senescing, latent infections will be activated first producing very large necrotic brown lesions that will be covered with black spores. Wind mainly and rain spreads the spores causing a plethora of smaller lesions. The development of large and multiple small lesions per leaf results in defoliation of leaflets or entire leaves. Rain is not necessary for Alternaria to develop. High relative humidity in the orchard is sufficient for causing latent infection and expression of symptoms. Although Alternaria late blight develops symptoms initially on the leaves, fruit can also decay from Alternaria infection of the hulls, but by the first harvest, not much disease will have developed.
Susceptibility to Disease
If you have Golden Hills expect to have earlier and more severe symptoms than symptoms on Kerman. In an experimental field at the Kearney Agric. Research and Extension Center 3.5 acres of pistachios are planted in a design of 2 rows each Kerman alternating with 2 rows of Golden Hills. After infection and symptom development severe defoliation occurred in the Golden Hills rather in Kerman trees (Figure 2). We do not know the basis for the higher susceptibility of Golden Hills to Alternaria late blight, perhaps leaves mature earlier as does the fruit in this variety and thus the disease shows up much earlier and defoliates the trees excessively.
Figure 2. Alternaria late blight symptoms (leaf lesions and defoliation) of Kerman (left) and Golden Hills (right) trees photographed at the same time (15 September 2023) in an Experimental pistachio orchard at the Kearney Agric. Research and Extension Center in Parlier, California.
Weather Conditions Favoring Disease
As mentioned above high relative humidity favors Alternaria late blight disease and that is why under normal dry conditions in the summer the disease develops in certain areas with micro-climatic conditions. For instance, in certain locations where mature orchards may have overlapping tree canopies, ventilation in the orchard is reduced and the relative humidity is high, regardless of the orchard irrigation (drip or micro-sprinkler irrigation). A good recommendation then for new plantings is to plant the rows of trees N to S orientation so that when trees develop their full canopy, the N to S prevailing wind will move easier and faster throughout the orchard than when the rows of trees are planted East to West. In mature orchards, a wise recommendation will be to do hedging of the trees to open a ventilation path between the tree rows of the orchard. A late rain that will provide excess water on mature leaves and nuts will be detrimental, causing severe Alternaria late blight. Severe Alternaria late blight on nuts will result in not harvestable nuts, and in fact, those harvested will have severe brown staining of the shell, thus significantly reducing the nut quality.
Fruit Load & Alternaria Late Blight
In previous research, we showed that Alternaria lesions develop first in leaves of shoots that bear fruit; in the contrary, Alternaria lesions developed much later in leaves of shoots not bearing any fruit. Several years ago, we quantified the Alternaria disease as relates to the fruit load of shoots in an orchard in Kings County and found out that by mid-August, 45% of the leaves in shoots with fruit had Alternaria lesions, while at the same time only 11% of the leaves had Alternaria lesions in shoot not bearing any fruit. Also, in “on” production years, shoots heavily loaded with fruit start resulting in some yellowing of leaves, due to the high nutrient demand of the sink (fruit load). Thus, those stressed leaves are more susceptible to be infected by A. alternata. Therefore, it is expected to have more severe Alternaria late blight disease in “on” production years than in “off” production years. We suggest keeping the pistachio’s nutrients in optimal levels throughout the growing season.
Management of Alternaria Late Blight of Pistachio
Efficacy trials over multiple years provided results that helped the registration of many fungicides for the control of Alternaria late blight. These fungicides belong in five FRAC groups, 3, 7, 9, 11, and 19. In addition, pre-mixture fungicides that contain combinations of these FRAC groups showed excellent efficacy against Alternaria blight (see fungicide efficacy tables at https://ipm.ucanr.edu/legacy_assets/pdf/pmg/fungicideefficacytiming.pdf). One of the fungicides (Viathon) is a mixture of tebuconazole (FRAC 3) and phosphonate (P07, 33).
Figure 3. Frequency of mutants (mutation G143A) conferring resistance of Alternaria alternata causing Alternaria late blight of pistachio to strobilurin fungicides (QoI, FRAC 11) in leaf samples collected from 20 orchards in San Joaquin and Sacramento Valleys in California.Figure 4. Frequency of mutants (all mutations) conferring resistance of Alternaria alternata causing Alternaria late blight of pistachio to dehydrogenase inhibitor fungicides (SDHI, FRAC 7) in leaf samples collected from 20 orchards in San Joaquin and northern Sacramento Valleys in California.
The discovery of new effective fungicides that belong to the FRAC groups mentioned above, usually leads to excellent disease control (rating 5) in the first 2-3 years. However, because most of these compounds are site specific and the resistance risk is high, eventually in subsequent years they become less efficacious in controlling Alternaria late blight. Despite the application of fungicides to control this disease, the frequency of Alternaria late blight samples brought to our lab by pest control advisors has increased exponentially in the last several years. This situation made us hypothesize that perhaps the resistance to the major fungicide FRAC groups might have become widespread. To find out if this contention was true, an extensive survey was initiated to determine what is going on. Samples of leaves (i.e. with lesions of Alternaria) (Figure 1) were collected from each of 20 orchards surveyed in southern and central San Joaquin Valley and the northern Sacramento Valley, brought to the laboratory and processed using the molecular qPCR procedure. Briefly, in this procedure lesions from each sample are blended, the DNA of Alternaria alternata is extracted, and by using specific primers, we can quantify the specific mutation(s) that confer resistance to the major groups of fungicides. For instance, the samples initially were processed to determine the levels of resistance to FRAC 11 fungicides, frequently used in pistachio orchards to control Alternaria late blight. Indeed, high resistance levels to strobilurins were detected mainly in the eastern side of south and central San Joaquin Valley and northern Sacramento Valley, while no or low levels of resistance were detected in orchards of the west side of the southern and central San Joaquin Valley (Figure 3). Similarly, when tests were done to determine the resistance levels to SDHI fungicides in diseased leaves from the 20 surveyed orchards, again high levels of resistance were detected (Figure 4). Interestingly, and very similarly to the resistance levels observed for stobilurin fungicides, the resistance levels to SDHI fungicides were lower in orchards of the west side of southern and central San Joaquin Valley (Figure 5). An explanation for these low resistance levels could be that usually growers in the Westside do not spray for Alternaria, since Alternaria late blight disease does not develop there (dry and windy, and “disease free” region!)
Figure 5. Frequency of mutants due to specific mutations conferring resistance of Alternaria alternata causing Alternaria late blight of pistachio to dehydrogenase inhibitor fungicides (SDHI, FRAC 7) in leaf samples collected from 20 orchards in San Joaquin and northern Sacramento Valleys in California. (Mutants with the H134R mutation are the dominant in all the fields, except, orchards #16, #17, and #18.) Interestingly, these orchards are located in the West Site/which is considered “disease-free” area.Figure 6. Fungicide efficacy trial in 2023 against Alternaria late blight of pistachio in an experimental pistachio orchard at the Kearney Agric. Research and Extension Center.
Alternaria late blight is very difficult disease to control because the pathogen is very prone to resistance selection to fungicides. In fact, because a major portion of Alternaria alternata population is resistant to two major FRAC groups of fungicides, a grower, a pest control adviser, or a consultant needs to be careful with the fungicide program to follow in order to achieve the maximum degree of disease control. Alternations of FRAC groups and pre-mixed fungicides can be used, along with at least one triazole (FRAC 3) in the program. In our 2023 fungicide efficacy trial experiment, we showed that the triazoles Cevya (i.e. mefentrifluconazole; registered) and the premixed Milbelya (i.e. mefentrifluconazole+ fluxapyroxad, FRAC 3/7; not registered yet), Luna Experience (i.e. tebuconazole+fluopyram, FRAC 3/7; registered) along with Miravis Prime (i.e. pydiflumetofen+fludioxonil, FRAC 7/12; registered) resulted in the least severe disease in an experimental orchard at Kearney Ag Research & Extension Center (Figure 6). This orchard has suffered from severe Alternaria late blight disease yearly.
Timing of fungicide Treatments
A grower or pest control advisor has three options regarding the timing of fungicide sprays for the control of Alternaria late blight. The first option is to apply one spray as an insurance coverage of the orchard. This should be done in the first week of July (Remember Independence Day!). Certainly, the decision of using one spray is based on historical data about Alternaria late blight disease. The second option would be 2 to 3 summer sprays. Sprays start beginning of June and end close to the end of July: 1st spray in early-June, 2nd spray in late June/early- July (best timing); and if high humidity is built up in the orchard due to the location and type of irrigation, then one more spray should be applied in mid- to late July. If any of these timings coincide with the timing of a NOW spray, you can combine these sprays in one, and thus save costs of tree spraying. Please do not spray in August and afterwards; instead of reducing the disease, you may create conditions to increase the blight.
The third option would be the use of a Disease Severity Value (DSV) model. For this there is a need to monitor leaf wetness and temperature in the orchard because combinations of those conditions determine when it is time for infections to occur, and therefore to spray. However, because not every grower has a weather station in the orchards and because this is more complicated to explain, in order to avoid confusion at this point, I will leave this discussion for a future issue of Pacific Nut Producer.
In summary, Alternaria late blight has become a major disease of pistachio in the last two decades, and it is a very difficult disease to control because the pathogen causing this disease, Alternaria alternata, is easily prone to fungicide resistance selection. The resistance levels were quantified in 2023 and found to be high and widespread among the main pistachio growing regions. Therefore, major caution is required when one decides on the fungicide program to control this disease. In addition, using an integrated control approach (cultural practices +chemical applications) will be the best option in controlling Alternaria late blight of pistachio. — By Themis J. Michailides, Boris X. Camiletti*, and Victor Gabri
It seems we are always on the verge of some type of apocalypse, and with the new carpophilus beetle invading California tree nut orchards, there is cause for concern. Watch this brief California Ag Network interview with UC Riverside CE Entomology Specialist Houston Wilson to learn more about this pest and how to stop it from damaging your crops.
New research at Oregon State University (OSU) suggests that filbertworm, the #1 insect pest of the hazelnut industry, is causing a lot more early season damage than growers have detected or attributed in the past. At a recent Nut Growers Society meeting, Pacific Nut Producer Editor Matthew Malcolm met with OSU hazelnut extension experts Nik Wiman and Betsey Miller to discuss amending filbertworm monitoring and management recommendations to improve control of the pest and resulting crop yields. Watch this brief interview and read more in Pacific Nut Producer Magazine.
In addition to good news of higher prices and a new hazelnut variety release from Oregon State University, hazelnut growers in attendance at the Nut Growers Society Winter Meeting were also discussing and expressing much concern over a new strain of Eastern Filbert Blight (EFB). Now spreading around the Willamette Valley of Oregon, this new strain appears to be no respecter of the long trusted EFB resistant varieties growers have been investing in. Pacific Nut Producer Editor Matthew Malcolm discussed this issue with the new Nut Growers Society President, Matt Miller, who happened to be among the first to discover this new EFB strain. Watch Miller’s interview and read more about it in Pacific Nut Producer Magazine.
Navel orangeworm (NOW) is most effectively controlled with the cultural practice of winter sanitation. Winter shaking almond trees to remove mummy nuts has proven to decrease next year’s NOW damage better than any other approach. The reason for this is clear. NOW overwinters as larvae in mummy nuts left in the tree after harvest and it is in these nuts the population carries over into the next season. Adult moths emerge in spring, mate, and lay eggs on mummies that are still in the trees as the females can’t find the new crop nuts until hull split. The second generation will then put direct pressure on the new crop nuts at hull split and the third generation will chew the nuts up during the harvest period.
The way almond prices have been going recently, there’s no doubt that everyone is going to have to spend dollars as wisely as possible for the foreseeable future. Although cleaning the trees of mummies during the winter isn’t cheap, it is the method of NOW control where you clearly get the most bang for the buck spent. We’re really playing a numbers game here, and this is one practice that is stacked in our favor by the biology of this pest.
For example, let’s assume a potential of 50 mummies per tree and 30 of them each have 1 NOW larvae. Half of those are female, and each female lays approximately 85 eggs. At harvest in late July we’re into the third generation, and for arguments sake let’s assume there’s no natural mortality.
Look at what could theoretically happen to the worm population in one Nonpareil tree with 30 infested over-wintering mummies and no control:
1st generation: 15 females x 85 eggs/female = 1,275 larvae
3rd generation: 54,188/2 x 85 = 2,302,990 larvae per tree at harvest!!!
(Thankfully, there IS natural mortality or else we’d be knee deep in worms!)
Now, look at the impact of a hull split spray aimed at the second NOW generation. We know that sprays give at best about 60 percent control. This reduces the population but is not nearly as good as sanitation as you will see.
2nd generation: 54,188 larvae x 40% survival after the spray = 21,675 larvae
3rd generation: 21,675/2 x 85 = 921,196 larvae per tree to attack the crop at harvest.
Now, look at what sanitation does in comparison. Start with the same 30 infested mummies per tree, then winter clean down to 2 mummies per tree. One is female, one is male.
1st generation: 1 female x 85 eggs/female = 85 larvae
3rd generation: 3,613 larvae/2 x 85 = 153,531 larvae per tree at harvest.
(If you can beat the 3rd generation by an early harvest you’re even further ahead.)
So, a hull split spray reduced the worm population by 60 percent, but sanitation by itself, without spraying, reduced the population by 94 percent! When more NOW larvae make it through the winter, more egg laying will occur next season regardless of what else you do. In relation to the number of mummies left in the tree, expensive chemical treatments next season will only slow the rate of worm damage increase.
If you have scarce dollars to spend on NOW control, spend them this winter when they will do the most good in a sanitation program. If the entire neighborhood works at this, the positive effect will be multiplied many times over for everyone. If you’ve got neighbors that don’t seem to get it, cleaning your orchard will still be a tremendous help to you. If you have no mummies, the first generation in the spring won’t be able to build up and establish a population in your orchard. You’ll benefit since they’ll have to fly in from the neighbors after hull split before they can begin to hurt your crop.
Be sure to finish the job by destroying the infested nuts once they’re on the ground. Mow and shred the mummies before March 1st so NOW moths don’t have a chance to emerge. When you’re enjoying mowing during bloom in the spring, take personal satisfaction in seeing the chips and pieces of almond fragments and mangled worm parts fly out from under your mower! — By Joseph Connell, UCCE Farm Advisor Emeritus, Butte County
In 2023, the cold, wet weather during bloom and leaf-out resulted in bacterial blast damage in many Sacramento Valley almond orchards. If conditions this year are cold and wet during bloom, we may see a recurrence of blast: make a plan now to keep blast damage to a minimum.
The Bottom Line:
Bacterial blast can be a problem when cold, wet conditions coincide with bloom or leaf-out.
Copper resistance is common in the pathogen: spraying copper is ineffective for preventing blast in many orchards.
Frost protection is the most economical prevention option and will help prevent damage from both frost and bacterial blast.
The antibiotic kasugamycin (Kasumin®) is effective for preventing blast when applied up to 7 days before cold, wet weather. It has received approval for availability this year under an emergency exemption registration (Section 18) but is not currently labeled for almond under a full registration (Section 3).The Details:
Pseudomonas syringae pv. syringae (Pseudomonas) is a bacterium which can infect all aboveground parts of an almond tree. If leaves, flowers, or spurs are infected, the resulting disease is bacterial blast. Pseudomonas is ubiquitous in orchards, so bacterial blast is likely to develop whenever environmental conditions are cold and wet, which favors this disease. Pseudomonas is spread by water hitting the trees, either from rain or sprinklers. If this wetting occurs at the same time as frost damage, Pseudomonas can enter the tree through cells damaged by freezing. Trees are especially susceptible to frost damage during bloom and leaf-out, when tender new growth is exposed to cold temperatures.
Preventing infection by Pseudomonas is the only way to control bacterial blast. Frost protection in an orchard is your best defense and your most inexpensive prevention strategy: if the trees are not damaged by frost, Pseudomonas will not be able to enter the tree to cause disease. As a second line of defense, research has shown that kasugamycin (Kasumin®) is effective for preventing bacterial blast when applied no more than 7 days before cold, wet weather. Note that kasugamycin is currently not labeled for use in almond, but has become available this season due to a Section 18 exemption. If the weather warrants treatment, kasugamycin can be used as a preventative spray up to two times during bloom. For this spray, complete coverage is crucial for control: all tender new growth must be covered in a protective layer of the antibiotic for it to be effective. Any tissue left uncovered will be unprotected.
What about copper? Current work by UC researchers shows that copper-resistance is common in Pseudomonas throughout the state. Many of the orchards heavily affected by blast in 2023 received multiple dormant sprays containing copper. In some cases, mixing mancozeb with copper may provide some level of control, but research shows that this mixture is not as effective as kasugamycin and the copper can cause phytotoxicity. — By Jaime Ott, UCCE Tehama, Shasta, Glenn, and Butte Counties
For more information on bacterial blast, check out these articles at sacvalleyorchards.com
Bacterial blast/canker: What do we know? – an update on the factors predisposing orchards to damage by bacterial blast, and more details about control strategies for bacterial blast
The Environmental Protection Agency (EPA) and California Department of Pesticide Regulation recently accepted a Section 18 petition to allow the use of Kasumin 2L (kasugamycin) to control bacterial blast (Pseudomonas sp) in almonds. The registration allows up to two applications under anticipated cold or freezing conditions on almonds at a use rate of 64 fl. oz. per acre from February 2, 2024, through petal fall. Application after petal fall is prohibited. Kasugamycin may only be used during bloom.
The approval applies to the counties of Butte, Calaveras, Colusa, Contra Costa, Fresno, Glenn, Madera, Merced, Placer, San Joaquin, Stanislaus, Sutter, Tehama, Yolo, and Yuba.
Growers interested in this application are heavily encouraged to reference the Almond Board of California’s Honey Bee Best Management Practices as well as the Quick Guide for Applicators (in English & Spanish) to ensure pollinator health is maintained. As stated in these practices, growers should only use applications when absolutely necessary and should only make applications in the late afternoon or evening, when bees and pollen are not present.
Please contact your local County Ag Commissioner’s office for further details if interested in using this product. Visit the California Department of Food and Agriculture website for a full list of County Ag Commissioners’ offices as well as contact information for each.
Every orchard grower understands the challenge of controlling fungal diseases. Although fungicide applications are the most effective strategy for disease management, the rise of fungicide resistance poses a significant risk to efficacy. Growers are often unaware they have a fungicide resistance issue until devastating diseases like blossom blight/brown rot, jacket rot and hull rot have caused substantial and irreversible damage to their crops. To protect tree nut health and maximize yield, growers should consider implementing proactive strategies to address fungicide resistance.
Understanding Fungicide Resistance: Fungicide resistance, as defined by the Fungicide Resistance Action Committee (FRAC), is an acquired reduction in sensitivity of fungi to specific fungicides. The key challenge is the genetic variability of fungal populations, which leads to fungicide resistance in certain fungi. Without a proactive approach, those resistant fungi can rapidly reproduce, causing widespread fungicide resistance across the orchard.
Spotting the Signs: Fungicide resistance is generally not found until there is an issue in the crop, but staying alert during the growing season can help growers spot signs of resistance. Avoid repeated use of fungicides from the same FRAC group, talk with neighbors about their treatment outcomes for different diseases and stay informed on resistant strains to avoid ineffective treatments.
Effects on Tree Nut Growers: Fungicide resistance poses a serious threat to growers’ bottom lines. With studies indicating potential preharvest losses ranging from 10% to 23% and postharvest losses of 10% to 20%, the stakes are high for growers across the industry.1 Implementing a proactive plan at the beginning of the growing season can give tree nut growers peace of mind when diseases, such as blossom blight/brown rot, jacket rot and hull rot, occur in their orchards.
Best Management Practices: To avoid the development of fungicide resistance, consider these best management practices:
Rotate fungicides with different FRAC groups to reduce the selection pressure on the fungi population.
Scout orchards daily for signs of disease development.
