- Author: Michael Hsu
Detection of fungus causing red leaf blotch spurs call for grower vigilance
Symptoms of red leaf blotch (RLB), a plant disease caused by the fungus Polystigma amygdalinum, have been observed for the first time in California across the Northern San Joaquin Valley.
Molecular DNA testing by the laboratory of Florent Trouillas, University of California Cooperative Extension fruit and nut crop pathology specialist, has detected P. amygdalinum. Pest identification was confirmed by the California Department of Food and Agriculture and U.S. Department of Agriculture.
The disease, named for the characteristic orange-to-dark red blotches that appear on infected leaves, is typically nonlethal for trees but has been a long-standing problem for almond-growing regions across the Mediterranean. Causing trees to lose their leaves prematurely, the fungal pathogen can significantly diminish crop yields in the current year and the next.
“It is one of the most severe diseases of almonds for Spain and the Middle East,” said Trouillas, an associate professor in the UC Davis Department of Plant Pathology. He recently co-authored an explanatory article on the UCCE San Joaquin Valley Trees and Vines blog.
With symptomatic trees seen in multiple orchards across Madera, Merced, San Joaquin and Stanislaus counties, Trouillas said RLB is already “somewhat widespread.”
“From the first observations so far, it seems like it affects some of the most-planted cultivars, like Nonpareil and Monterey,” he added. “We've observed it in a diversity of cultivars already.”
UC Cooperative Extension specialist urges taking preventive measures
According to Trouillas, RLB caused by P. amygdalinum is “highly specific” to almond trees, and generally only affects their leaves. Infection typically happens at petal fall, when small leaflets are first emerging and most susceptible to disease. After the pathogen's latent period of about 35 to 40 days, the first symptoms appear – small, pale-yellow spots on both sides of the leaves.
Those blotches become yellow-orange and then reddish-brown in the advanced stages of the disease during June and July. Now, with RLB symptoms becoming more prominent, Trouillas and UC Cooperative Extension advisors across the Central Valley have seen an uptick in calls.
“PCAs [pest control advisers] have been confused because they've never seen anything like this,” said Trouillas, noting that the yellow-orange-red blotches are symptoms unique to RLB and cannot be confused with other known almond diseases.
Applying fungicides after RLB symptoms appear is ineffective, Trouillas said. The best thing growers can do at this point is to report symptomatic trees to researchers so they can track the prevalence and distribution of the disease.
Growers who see signs of this new disease in their orchard should contact their local UC Cooperative Extension farm advisor.
Preventive measures are the best way to manage RLB, Trouillas said. He urges concerned growers to think ahead to next winter/spring and plan for fungicide applications at petal fall and – if rains persist – also at two weeks and five weeks after petal fall. Fortunately, those are the same three key timings for managing other diseases, like shot hole and almond anthracnose.
“Because RLB is something that is introduced and potentially aggressive, it will be important for growers to keep that in mind next year and be on schedule for next year's spraying program,” Trouillas said.
Additional information on RLB can be found at https://www.sjvtandv.com/blog/first-detection-of-red-leaf-blotch-a-new-disease-of-almond-in-california.
- Author: Steven Swain
Sudden oak death (SOD) is a disease syndrome that has killed millions of native oak trees (Figure 1) along the west coast of the United States, from Big Sur in California up to Southern Oregon. The disease may involve several organisms, but its main driver is the fungus-like organism (known as water mold), Phytophthora ramorum. This plant pathogen is spread in the springtime by windy rainstorms. It infects the bark of oak trees, frequently creating bleeding trunk cankers that interfere with water uptake and sugar transport.
Death of SOD-infected trees can be accelerated by attacks from bark and ambrosia beetles. In the absence of beetle attacks, infected oaks may take years to die.
Many common disorders (other than P. ramorum infections) can cause damage that resembles SOD, so laboratory testing is needed to confirm the diagnosis.
Sudden Oak Death Host Range
Sudden oak death isn't always sudden, nor does it infect just oaks. The potential host list of P. ramorum includes hundreds of plant species, many of which are natives of California's woodlands and forests. In most of these host species, the pathogen simply causes small necrotic spots on the leaves (Figure 2). In a few instances, notably in certain susceptible Rhododendron cultivars, the disease may progress from the leaves down into the stem and kill the plant.
Phytophthora ramorum has evolved as a foliar pathogen, primarily spreading from the leaves of infected hosts such as bay laurel (Umbellularia californica), tanoak (Notholithocarpus densiflorus), and rhododendrons. Therefore, it does not spread easily to other plants from oak trunk cankers. Thus, when oaks become infected, they are referred to as terminal hosts. Tanoak acts as both a foliar host, spreading spores from its leaves in wet and windy weather conditions, and a terminal host, developing cankers that are almost always lethal to the infected plant (Figure 3). Perhaps because infected tanoak leaves rain spores onto their own trunks, their SOD survival rates are among the lowest of all trees that may become afflicted by the disease in North America.
Despite its fearsome reputation, SOD doesn't always kill infected oaks. Valley oak (Quercus lobata), blue oak (Q. douglasii), and Oregon white oak (Q. garryana) are not known to develop cankers in nature, while coast live oak (Q. agrifolia), black oak (Q. kelloggii), Shreve oak (Q. parvula var. shrevei), and interior live oak (Q. wislizeni) are considered susceptible. Recent studies have demonstrated that there may be considerable variance in some coast live oaks, with measured resistance varying from about 16-40%. Many of these resistant or tolerant trees do become infected but are able to defeat the pathogen before trunk cankers enlarge to life-threatening sizes. Casual observations suggest that resistance levels seen in coast live oaks are likely similar to those in Shreve oak and black oak populations. Because interior live oak trees grow in comparatively hotter, dryer, environments than the other susceptible oaks, they almost never become infected, so resistance levels have not been studied. Tree age seems to play a role too. Oak trees under four inches in trunk diameter at chest height are not typically susceptible to infection by P. ramorum.
Treatment Approaches
Many different treatment approaches have been trialed, a few of which have shown promise.
Prevention
Potassium phosphite compounds (AgriFos, Reliant, Garden Phos, etc.) work best as preventive treatments. Most of the efficacy achieved by these compounds appears to be the result of stimulating the trees' natural defensive systems, although individual oaks vary widely in their immune responses. Thus, the application of potassium phosphite compounds is a bit of a gamble as to whether it will actually help an individual tree. Trees that are already showing symptoms of infection when treated have a significantly lower survival probability than trees that appear healthy during treatment.
There are two different recommended application methods for potassium phosphites—surface sprays made directly to the bark (using a surfactant such as Pentrabark) and trunk injections. Some applicators have claimed increased efficacy from the trunk injections, albeit at the cost of potential damage to the tree from wounding. Others have claimed equivalent efficacy with repeated bark sprays, when carefully timed. Application of calcium to the root zones of oaks treated with potassium phosphite has been shown to further improve resistance rates.
Potassium phosphite treatments should be made in the spring and fall, regardless of the application method used. This is because the uptake of potassium phosphite by the tree is dependent on high transpiration rates. High transpiration rates in turn depend on both adequate available soil moisture, and warm and sunny weather, preferably with a light breeze.
Another preventive treatment option that has proven to be effective is removal of foliar hosts that are near highly valued oak trees (Figure 4). Removing bay laurels that have foliage within about 30 feet of an oak trunk decreases the chances of that oak becoming infected. Removal is especially effective for small, understory foliar hosts, such as young bay laurels, poison oak, and rhododendron. This is not a recommendation for the wholesale removal of bay laurel trees, which are important parts of the California forest ecosystem and should be retained where appropriate. This management approach should only be adopted after careful consideration of the ecological function of the forest or woodland as a whole.
Management of Active Infections
Bark scribing, or cutting away the outer portion of infected bark to let the infection site dry out, was previously touted as an effective treatment for infected oaks. However, rigorous testing has since shown that it does not significantly increase the odds of an oak surviving. Similarly, the application of whitewash to tree trunks has not been shown to make measurable improvements to survival of treated oaks as compared to no treatment at all.
Mefanoxam (Subdue Maxx, Stergo MX, etc.) is a fungicide with a proven record of suppressing Phytophthora activity. It has been used to successfully treat infected plants, keeping them alive as long as treatment continues, typically over the span of a year or two. However, in most cases, once treatment stops, Phytophthora begins growing again within several months, and the infected plants will eventually succumb to the disease. While mefanoxam fungicides may be effectively used in certain limited situations, they should not be considered effective curative treatments. Resistance to this active ingredient has developed repeatedly, rendering it unacceptable for long-term use.
In summary, no silver bullet exists for preventing SOD, and there are no effective tools that will reliably save a tree that is already showing symptoms of infection. This does not mean that any oak that exhibits SOD trunk cankers or bleeding symptoms will die. Bleeding from the bark is a normal response to substantial damage, whether from a pest, pathogen, or, sometimes, even mechanical damage. Even if bleeding is the result of infection by P. ramorum, symptomatic trees have recovered with no intervention, usually in association with a few dry years. The most effective tools for managing SOD are preventive, in nature. For more information and best management practices, please see the California Oak Mortality Task Force website at: https://www.suddenoakdeath.org.
- Author: Emily C. Dooley, UC Davis
Pathogen native to U.S. but had not infected pines until recently
Fungal pathogens that cause die-back in grape, avocado, citrus, nut and other crops has found a new host and is infecting conifer trees causing pine ghost canker in urban forest areas of Southern California.
The canker can be deadly to trees.
Scientists from University of California, Davis, first spotted evidence that the pathogens had moved to pines during a routine examination of trees in Orange County. Over four years, they found that more than 30 mature pines had been infected in an area of nearly 100 acres, according to a report in the journal Plant Disease.
Akif Eskalen, a professor of Cooperative Extension in the Department of Plant Pathology at UC Davis, suspects drought and other stress conditions brought on by climate change weakened the tree species, making it more susceptible to new threats.
“We have been seeing this on pine trees for the last several years,” he said. “Our common crop pathogens are finding new hosts.”
Pine ghost canker – caused by the fungal pathogens Neofusicoccum mediterraneum and Neofusicoccum parvum – usually infects the lower part of a tree's canopy, killing branches before moving on to the trunks. This dieback in some cases can be deadly.
Points of entry
The pathogens infect a tree by entering through wounds caused by either insects such as red-haired pine bark beetles or pruning – meaning trees in managed or landscaped areas could be at risk. Another route is via tiny natural openings known as lenticels that fungi can make their way through, said Marcelo Bustamante, a Ph.D. candidate in Eskalen's lab who is first author on the paper.
Spores from the fungi can disperse and the higher the prevalence means an increased chance of transmission. Rain, irrigation water and humidity by fog can trigger the right circumstances for the spores to spread, he said.
“The detection of these pathogens in urban forests raises concerns of potential spillover events to other forest and agricultural hosts in Southern California,” Bustamante and others wrote in the report.
Dead branches can indicate a canker. Detecting the fungi is not an emergency but “people should keep an eye on their plants when they see abnormalities,” Eskalen said.
Cankers are localized areas on stems, branches and tree trunks that are usually dead, discolored and sunken. On bark, the spores can look like strings of discolored dots.
The lab has posted a brochure bout how to best manage wood canker diseases.
Tips include:
* Keep your trees healthy: Proper irrigation and maintenance will keep trees strong.
* Prune dead branches to reduce sources of infestation.
* Avoid unnecessary pruning; perform structural pruning only.
Karina Elfar, Molly Arreguin, Carissa Chiang, Samuel Wells and Karen Alarcon from the Department of Plant Pathology contributed to the paper, as did experts from Disneyland Resort Horticulture Department, State University of New York's College of Environmental Science and Forestry, UC Irvine and UC Los Angeles.
- Author: Lauren Fordyce
With Respiratory Syncytial Virus Infection (RSV) on the rise, and Covid-19 and the flu remaining constant worries, disinfectant products are more and more likely to be used in the home, office, school, restaurant, and other public areas. Though these products are useful in reducing harmful pathogens, they are also capable of harming us when used incorrectly.
You may not think twice when spraying a surface with a disinfectant or using a disinfectant wipe without wearing personal protective equipment (PPE). Disinfectant products ARE pesticides, so check the label to see if you should be wearing gloves or other protective equipment.
To learn more about safe use of disinfectants and wipes, watch the recording from our most recent webinar with Meredith Cocks from the National Pesticide Information Center (NPIC).
Be sure to check out the UC IPM webinars planned for 2023 and register for these free, educational events!
- Author: Emily C. Dooley, UC Davis
E. coli and Salmonella are rare in wild birds, Campylobacter more common
Concerns over foodborne risk from birds may not be as severe as once thought by produce farmers, according to research from the University of California, Davis, that found low instances of E. coli and Salmonella prevalence.
While the research found that the risk is often low, it varies depending on species. Birds like starlings that flock in large numbers and forage on the ground near cattle are more likely to spread pathogenic bacteria to crops like lettuce, spinach and broccoli, according to a study of food safety risk and bird pathogens from the University of California Davis. In contrast, insect-eating species were less likely to carry pathogens.
The findings, published in the journal Ecological Applications, suggest that current practice of removing bird habitats around produce growers' farms over concerns the animals could bring foodborne pathogens into their fields may not solve the problem.
“Farmers are increasingly concerned that birds may be spreading foodborne diseases to their crops,” said Daniel Karp, the senior author on the study and an assistant professor in the UC Davis Department of Wildlife, Fish and Conservation Biology. “Yet not all bird species are equally risky.”
Only one foodborne disease outbreak in produce has been conclusively traced to birds: a Campylobacter outbreak in peas from Alaska. While the bacteria can cause diarrhea and other foodborne illness in humans, it's less of a concern to growers than E. coli and Salmonella, which have been responsible for multiple outbreaks across the nation.
In this study, researchers compiled more than 11,000 bacteria tests of wild bird feces and found that Campylobacter was detected in 8 percent of samples. But pathogenic E. Coli and Salmonella were only found in very rare cases (less than 0.5%).
In addition to the bacteria tests, researchers conducted roughly 1,500 bird surveys across 350 fresh produce fields in Western states and collected more than 1,200 fecal samples from fields. They then modeled the prevalence of pathogens in feces, interactions with crops, and the likelihood of different bird species to defecate on crops to determine risk.
Insect-eating birds pose lower risk
Based on the data, insect-eating birds, such as swallows, present a lower risk, while birds that flock near livestock, such as blackbirds and starlings, are more likely to transmit pathogens.
The data can help the agricultural industry determine risk and take action, such as separating produce crops from cattle lands. They also don't need to treat all birds the same.
“Maybe farmers don't need to be quite as concerned about all types of birds,” Karp said. “Our data suggest that some of the pest-eating birds that can really benefit crop production may not be so risky from a food-safety perspective.”
Removing habitat can backfire
This study and the authors' prior work indicate that removing habitat around farms may actually benefit the species that pose more risk and harm the beneficial, pest-eating ones that are less risky to food safety. This is because many prolific insect-eaters may visit crop fields to eat pests but need nearby natural habitats to survive. In contrast, many of the bird species that most commonly carry foodborne pathogens readily thrive on both cattle farms and produce farms without natural habitat nearby.
Other findings
Insect-eating birds that forage in the tree canopy pose minimal threat because they are less likely to carry foodborne pathogens and come into direct contact with produce. They can also be valuable parts of the ecosystem, particularly if they eat pests that can harm crops. Installing bird boxes could attract the pest-eaters, as well as help with conservation efforts.
“We basically didn't know which birds were problematic,” said lead author Olivia Smith, a postdoctoral researcher at Michigan State University who was at University of Georgia when the paper was written. “I think this is a good step forward for the field.”
Additional co-authoring institutions include James Cook University, UC Berkeley, UC Riverside, University of Kentucky, University of Texas, Virginia Polytechnic Institute and State University, Washington State University, BioEpAr, The Nature Conservancy and Van Andel Institute.
The research was funded by the United States Department of Agriculture and the National Science Foundation.
