February 2022 Almond Orchard Tasks

By Brent Holtz, Ph.D.

Also published in Pacific Nut Producer

Bee management, bloom diseases, and frost protection are typical concerns for February. Flowering weeds should be mowed in the orchard, both for frost protection and to avoid competing with almond flowers for bee visitation. Beekeepers usually manage their bees through hive placement and supplying food and water when needed. Bee management for growers includes protecting bees against pesticide exposure, for there is growing evidence that certain pesticides applied in combination during bloom can kill or reduce bee larvae development.

The Almond Board of California recently published “Honey Bee Best Management Practices for California Almonds” where practices that minimize bee exposure to pesticides while maximizing bee health are discussed and demonstrated. Joe Traynor, from Scientific Ag Company, has pointed out that bee activity may be enhanced by placing bees where they will get the most sunlight, especially early in the morning. It is recommended that if a fungicide application becomes necessary for disease control during bloom, applications should be made in late afternoon or evening (http://ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf). Avoid applying insecticides during bloom while bees are in your orchard. Try to avoid tank mixes that contain a lot of different products including ‘the kitchen sink,’ for it is difficult to determine how all these product interactions can affect pollen used for bee growth and development. Fortunately, you have plenty of options to control insect pests, and to apply foliar nutrients, after bloom when bee hives are no longer in your orchards (www.ipm.ucdavis.edu).

Almond trees are susceptible to blossom and foliar diseases when it rains at bloom time. Many of these diseases can be effectively managed with the use of properly timed fungicides. The decision, however, of when to spray and what fungicide to use can be quite difficult. The San Joaquin Valley typically receives a low amount of precipitation, but in many cases, we cannot predict when and how much it is going to rain. We often receive rain during the bloom period that can result in favorable conditions for several plant pathogenic fungi that can cause several diseases of almond blossoms. The fungi that cause these diseases are usually present in almond orchards, at higher or lower inoculum levels depending on the previous year’s disease incidence and current environmental conditions.

Generally, a good disease management program is based upon a wise choice of fungicides and good timing and coverage. Growers should assess the diseases present in their orchards and select materials carefully. Not all fungicides are equally effective against all diseases. It is a good idea to use more than one kind of fungicide to obtain a broader spectrum of activity. Please read the on-line publication “Fungicide efficacy and timing for deciduous tree fruit and nut crops and grapevines” that can be found at the UC IPM website at http://ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf. This is the ‘Bible’ of bloom and foliar disease management. Group numbers have been assigned to each fungicide based on its mode of action by the Fungicide Resistance Action Committee (FRAC) (http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. Resistance to fungicides can develop over time with repeated use of any on FRAC Group, thus, we need to rotate the fungicides we use. When making fungicide applications, keep track of their FRAC numbers, and, if possible, make only one application each of FRAC numbers 1, 3, 7, 9, 11, and 17 per season. After using one of these fungicides, rotate to another number, don’t use the same number for two consecutive sprays but be sure the fungicide selected has activity against the disease. For fungicides with other FRAC numbers, make no more than two consecutive applications before rotating to a fungicide with a different FRAC number. Under low rainfall forecasts, a single, delayed-bloom application of a newer pre-mixture fungicide such as a FRAC Group 3/11, 3/7, 3/9, or 7/11 will provide “reach back” and protection. A delayed bloom application is done at 40% open flowers. Under high disease pressure, a pink bud (5% open flowers) followed by a full bloom (80% open flowers) application is suggested (see below). Dr. Jim Adaskaveg, has done a great job authoring the ‘Fungicide Efficacy’ document and has put together a color guide (http://ucanr.edu/sites/sayalmond/Almond_Publications/) that shows the symptoms of different diseases.

I was recently notified that the European Union (EU) has lowered the maximum residue limit (MRL) on Luna fungicides (fluopyram) from 0.05 ppm, which is the same as in the US, to 0.03 ppm. In order to comply with the MRL in the EU, Bayer is recommending that Luna only be used during bloom sprays up to 30 days after full bloom. Ongoing research will determine how late we can safely spray Luna in the future.

Bacterial spot, Xanthomonas arboricola pv. pruni, was found and identified in the Northern San Joaquin Valley in 2013. Symptoms of infected nuts include the production of an amber colored gum from spots on the hull. We initially recommended trying to reduce inoculum levels by defoliating leaves with zinc sulfate in the fall, destroying mummies, and spraying fall, dormant, or delayed dormant copper treatments to reduce overwintering inoculum.

Dr. Jim Adaskaveg conducted field trials on bacterial spot primarily on ‘Fritz’ over the last several years that included dormant and in-season applications. Late-dormant treatments (late January) with copper or copper-mancozeb significantly reduced the incidence of disease. Early dormant treatments that were applied in mid-November or mid-December were effective in low rainfall years. In-season treatments starting at full bloom through petal fall were most effective when timed around rain events and before temperatures started to increase above 77ºF (25°C) in the springtime. Copper treatments were applied by halving the rate with each successive application (e.g., 1, 0.5, 0.25 lbs MCE/A – metallic copper equivalent / acre). No copper phytotoxicity or only minor leaf spotting with no leaf drop was observed after four applications when copper rates were successively reduced. All copper products significantly reduced disease. Among products evaluated, however, Kocide 3000, Badge X2, Champ-Ion2+, and Cuprofix were more effective than Cueva. A single delayed dormant (copper-mancozeb) and a single in-season treatment (copper alone or copper+mancozeb) in mid-March timed before rainfall and warm conditions provided excellent control and was equivalent to five applications (delayed dormant and four applications in 10- to 14-day intervals starting at full bloom) using the same chemicals. For organic growers, Actinovate and Serenade also reduce bacterial spot, although not as effectively as copper products. For better control, Serenade should be mixed with copper. Avoid high angle-sprinkler irrigation, continued wetting of the foliage and fruit into the season will allow bacterial spot to increase in incidence and negate your springtime program.

Every year is different and IPM practices should be followed to determine the best timing and material to spray. Growers often concentrate their control measures on brown rot sprays at first bloom and often neglect their scab and Alternaria sprays that are typically done at 2-5 weeks after petal fall. Everyone is anxious to spray Nonpareil at pink bud which is the least important time to spray this most disease-resistant variety, yet cultivars Monterey and Carmel are often not sprayed for scab.

Two sprays are made for brown rot control under high disease pressure. The first is usually done at pink bud (5-10 % bloom) using a systemic fungicide such as a DMI (FRAC 3) or AP (FRAC 9). The second spray should be done near 80% (full bloom) or 7-10 days after the first spray. This is the most effective brown rot spray program! Depending on the weather, a third spray may be necessary for protecting against jacket rot and green fruit rot caused by Monilinia, Botrytis, and Sclerotinia species, as well as other diseases (e.g., shot hole) if rains persist and two weeks of protection have passed. This application can be with a systemic or a contact fungicide. Try to reduce the potential of resistance by using a multi-site compound (such as ziram, chlorothalonil, captan, or mancozeb).

Application techniques are important. Ground applications are better than air; but care must be taken that both are applied correctly. In general, use properly calibrated and directed nozzles while spraying and maintain a slow ground speed (<2.5 mph). The brown rot fungus (Monilinia laxa) attacks the tree by invading the stamens and pistils of the flower when it is open. From there, the fungus can move into and kill the spur or shoot. Young fruit are also susceptible in early spring and infection of fruit may extend to spurs and shoots. Although all cultivars of almond are susceptible to brown rot, they vary in their degree of susceptibility, Nonpareil, Peerless, and Aldrich are less susceptible, whereas Butte, Wood Colony, Winters, Mission, and Livingston are some of the more susceptible varieties, followed by Sonora, Fritz, Monterey, and Carmel. Varieties that are susceptible to green fruit rot or jacket rot are Butte, NePlus Ultra, Merced, Carmel, Price, Wood Colony, or any variety with tight fruit clusters. If bloom is extended and the weather is wet and rainy, no more than ten days should elapse between treatments.

The shot hole fungus (Wilsonomyces carpophilus) is notoriously more prevalent in wet years. This fungus requires water for all its activities, so periods of extended rainfall create a situation that favors shot hole disease epidemics. The fungus can cause lesions on leaves and fruit, but most of the time it infects the leaves as they emerge from the leaf bud. Leaf infections may lead to defoliation, which usually occurs in early spring. Shot hole infections of young fruit, shortly after they emerge from the jacket, can cause the fruit to drop. As fruits enlarge, shot hole infection results in a lesion, but the fruit no longer fall. About the first of May, when the embryo of the nut begins to grow, the hull becomes resistant to infection and no further lesions develop. Shot hole is usually controlled by fungicide applications after bloom (when leaves emerge after bloom), usually from petal fall to two weeks after petal fall. An IPM strategy for shot hole control is to monitor orchards in the fall and spring for shot hole lesions and fruiting structures. Fruiting structures appear in the center of leaf lesions as small black spots (sporodochia) and can be seen with a hand lens (www.ipm.ucdavis.edu). If fruiting structures are present in leaf lesions in fall, then a treatment the following spring should be applied at leaf emergence (sometimes this can be concurrent with bloom). If fruiting structures are not present, you can hold off the petal fall spray and monitor leaves in the spring for lesions. As soon as fruiting structures are evident, however, apply a fungicide as long as conditions are conducive to disease development (wet). If fruiting structures are not present, delay treatment until they are. Zinc Sulfate (20-40 lb/acre) applied in late October to early November will hasten leaf fall and prevent shot hole inoculum from increasing with late fall rains.

Scab (Venturia carpophila; formerly Cladosporium carpophilum or Fusicladium carpophilum) was effectively controlled with the strobilurin or QoI (QoI-quinone outside inhibitor) fungicides (Abound, Gem), but resistance to these fungicides has developed and we now recommend not using FRAC Group 11 fungicides exclusively unless in pre-mixtures or tank mixtures and in orchards without known resistance. Dr. Adaskaveg has developed a three-spray strategy for scab control that includes a delayed dormant application of copper-oil or chlorothalonil-oil, a two-week after petal fall spray that includes chlorothalonil (Echo, Bravo, Equus; FRAC M5), and a 5-week after petal fall spray that includes Captan (FRAC M4), Ziram (FRAC M3), or pre-mixtures of DMI (FRAC 3), SDHI (FRAC 7), or QoI (FRAC 11) fungicides. Ph-D (FRAC 19) can also be used in tank mixtures. Mixtures and pre-mixtures include for example 3/9, 3/11, 3/19, or 7/11 fungicides. Other fungicides such as maneb (recently cancelled) can be used until supplies are exhausted. The mancozeb product (FRAC M3) was registered as Manzate (http://www.cdms.net/LDat/ld72F000.pdf) in 2012. Syllit (FRAC U12; unknown mechanism) was federally registered in Dec. 2012 and is currently registered in California. All of the multi-site mode of action fungicides will have a low chance of resistance developing to them.

Recent work by Dr. Adaskaveg also has shown that delayed dormant applications of chlorothalonil and oil are even better than copper and oil at reducing scab inoculum well into the spring season. This will reduce in-season sprays and, if Alternaria leaf spot is also a problem, the in-season sprays for scab and Alternaria will be aligned (late spring). Venturia (i.e., Fusicladium) causes greasy dark spots on fruit, leaves, and green shoots. The shoot lesions are the overwintering sites for the fungus and the source of new spores in the spring. The disease causes no apparent damage to the fruit, but leaves may fall prematurely. Scab can completely defoliate a tree in a short time. All cultivars appear susceptible, but Carmel, Peerless, and Monterey are especially vulnerable. One of the more complicated aspects for managing this disease is that it is slow to develop and symptoms apparently develop all at once. When this happens, most growers and PCAs want to start treating, however, it is very difficult to manage the disease at this stage and use of single-site mode of action fungicides may lead to resistance due to high inoculum levels. Under these conditions only multi-site mode of action materials like sulfur or captan should be used.

Rust (Tranzschelia discolor) can also cause defoliation. Both rust and scab are favored by high humidity and are usually worse in years with late spring rains. Irrigation practices that create humid orchard conditions for long periods of time favor both diseases. Like scab, rust usually appears in late spring or early summer. The fungus attacks leaves but not fruit. On leaves, it produces small, bright yellow angular spots on the upper leaf surface, and reddish orange pustules on the lower leaf surface. The first fungicide application for rust control should take place as soon as disease symptoms are first observed in spring or early summer (e.g., if 1 out of 100 randomly collected leaves in a block or 1% of the leaves show symptoms, then an application is recommended). Treatments could be integrated with an Alternaria control program. In orchards with a history of rust, applications should be made before symptoms are observed. Maneb was a highly effective treatment (registration recently canceled) but is now replaced with mancozeb (Manzate). This fungicide and chlorothalonil (Bravo, Equus, Echo) can be applied until 5 weeks after petal fall and followed up again with another fungicide (sulfur, DMI, or QoI - FRAC Groups M2, 3, 11) 10 weeks after petal fall (http://www.ipm.ucdavis.edu). Usually, only one to two applications of FRAC 3 and 11 fungicides are necessary for a high level of disease control, but sulfur may need multiple applications due to its short residual activity. Zinc sulfate (20-40 lb/acre) applied in late October to early November will hasten leaf fall and prevent rust inoculum from increasing.

Alternaria leaf spot is becoming more problematic particularly in humid locations in the San Joaquin Valley; I have observed severe infections in humid pockets in Kern County

and in the northern Sacramento valley. Alternaria first appears in early summer on exposed leaves and is usually more severe on open-trained and spreading-canopy trees planted in an east/west direction. Alternaria can cause large lesions on leaves and can cause severe defoliation. Varieties that are more susceptible include Carmel, Sonora, Monterey, Winters, and Butte, but it has also been observed on Nonpareil, Price, Fritz, and Peerless. If your orchard is in a problem area or monitoring indicates the presence of Alternaria, begin late-spring treatments about mid-April (5 weeks after petal fall) followed by as many as three or more late-spring applications. Dr. Adaskaveg recommends using iprodione (Rovral, Nevado) in the southern almond growing areas where it warms up sooner than central and northern areas followed by other fungicides with differing modes of action. The registration restrictions of iprodione (limit at 5 weeks after petal fall) however, prohibit optimal timing and thus, the performance of iprodione in managing this disease has not been highly effective. Dr. Adaskaveg has successfully used an Alternaria Disease Severity Model (DSV) to predict one to three infection periods from May to late June or early July. Because Alternaria resistance to QoI has been documented, do not use FRAC 11 fungicides in these orchards. Currently, FRAC 3 (e.g., Quash, Tilt) and FRAC 19 (e.g., Ph-D), as well as mixtures and pre-mixtures (Inspire Super, Luna Sensation, Luna Experience, Quadris Top, Quilt Xcel) are registered and highly effective when used in a two- to three-spray program. (Remember, however, that Luna products should not be used later than 30 days after full bloom to ensure residues do not exceed MRLs in the European Union or EU).

An integrated control program that also includes reducing orchard humidity is critical: this could include planting orchards north-south orientation to allow for prevailing wind movement through the orchard, orchard hedging to increase air movement through the canopy, adding gypsum to improve water penetration, and monitoring leaf stem water potentials to avoid over-irrigation. Partial deficit irrigation may be desirable in Alternaria plagued orchards.

An extremely damaging fungal disease, anthracnose (Colletotrichum acutatum) can be severe in warm, wet springs, with average daily temperatures above 63F. We saw more anthracnose in 2011 because of all the spring rain and an El Nino spring. Usually, we don’t see anthracnose commonly in the San Joaquin Valley. On fruit, anthracnose can cause deep crater-like lesions; the affected area turns a rusty-reddish brown, older fruit often gum profusely, and the nut meat is usually destroyed. The fungus can also infect flowers during warm wet rains and is reported to invade the wood, and the branches upon which infected fruit reside weaken and die. In addition to destroying the crop, long-term damage and weakening of the tree may occur. Usually with warming temperatures in April and May, a good scab control program will usually control or reduce anthracnose. Orchards that have a history of anthracnose should be treated during bloom, starting even at pink bud (your brown rot spray to protect blossoms), to help reduce inoculum build-up. Ideal conditions for disease are warm rains, and protecting trees before every rain is necessary for ideal control. Many cultivars are susceptible to anthracnose; Thompson, Merced, Price, Peerless, Winters, Monterey, NePlus Ultra, Fritz, Carmel, and Butte appear quite susceptible; while Harvey, Padre, and Mission are moderately susceptible. Nonpareil is considered to be less susceptible. In orchards that have a history of anthracnose, apply fungicide sprays every 10 to 14 days if rains persist after bloom. Late spring rains may necessitate additional applications into May. Alternate fungicides as we have discussed using FRAC numbers (3, 7/11, 11, M3, M4, M5) rotating materials starting at pink bud using azoxystrobin (Abound) or a DMI fungicide, followed by a 7/11 premixture (e.g., Pristine, Merivon, Luna Sensation), a 3/11 premixture (Quilt Xcel, Quadris Top), followed by a tank mix of chlorothalonil, captan or mancozeb with thiophanate-methyl (Topsin-M), a DMI, or a QoI fungicide. Pruning out dead, infected wood reduces inoculum. If sprinkler irrigation is practiced, use low-angle nozzles to prevent the tree canopy from being wetted by sprinklers. Fortunately, we have new fungicides that were recently registered such as mefentrifluconazole (Cevya) and premixtures such as difenoconazole-pydiflumetofen (Miravis Duo) that have built-in resistance management mechanisms.

Frost can occur, so keep vegetation mowed and avoid working the soil from bloom through the end of March. The UC Almond Production Manual has a chart that displays temperatures at which cold induced damage can occur for different varieties. Freezes can occur through early April, so be prepared to run water if you can. Bacterial blast and canker develop under cold, wet environments. Last year, there was an increased incidence of blast especially in northern California almond production areas. Blast symptoms are often confused with brown rot blossom blight. The difference is that blossom blight often has tan to brown fungal sporulation on the dead flower parts. The Almond Alliance of California and Dr. Adaskaveg have requested the renewal of the emergency registration (Section-18) for kasugamycin (Kasumin) for Feb.-April 2022, similar to the previous two years. A full registration on almond is pending for early 2022. The antibiotic is not used in animal and plant medicine and belongs to its own FRAC Code 24 that is different from other antibiotics. Kasugamycin should be applied before frost events, especially on varieties susceptible to blast such as Independence, Aldrich, Monterey, and Fritz. Frost or frozen water causes tissue expansion and injury that is subsequently colonized by the ubiquitous epiphytic bacterium Pseudomonas syringae. In California, the pathogen is mostly resistant to copper. Kasugamycin is compatible with fungicides, and tank mixtures can be used during bloom and petal fall if cold wet weather is forecasted to minimize application costs. Integrated strategies are best for managing diseases like bacterial blast, thus, frost protection irrigation and the use of kasugamycin should be highly effective.

A pilot program for weekly regional forecasting of several almond foliar diseases will be initiated, and notifications of infection events will be made to the almond industry. Alternaria leaf spot, anthracnose, bacterial blast and spot, green fruit rot, scab, and rust will be included in this program. This project will be done in cooperation with the Almond Board of California, Semios In-Canopy Wireless Network, University of California Cooperative Extension, and the University of California. Farm advisors will also provide this information in newsletters or e-mail alerts to growers and PCAs.

Good Luck and Stay Safe!!!