David Doll0 commentsJune 9, 2012Diseases

Summer Disease Control: Should I Spray Again?

This past week, rain showers hit across the state, extending as far south as the Madera/Fresno area. In the Merced area, rain was variable, with most reports indicating about a 0.5″ of rainfall.I have had a few questions regarding the the need for another fungicide spray for the summer diseases of Rust and Alternaria. The following conditions and considerations should be kept in mind when deciding the need for another spray: Orchard History. If defoliation occurred last year due to these pathogens, there is a good chance that the over-wintering inoculum levels are high, increasing the chance for an epidemic this year. This factor remains important even if sprays were applied earlier in the season. Conditions post rain event. The hours and days after the rain were windy and warm, which dries the orchard. These conditions helped reduce the duration of leaf wetness, and canopy humidity, creating an environment that would be less favorable for disease. Timing of last fungicide spray. If a fungicide was applied in late May due to a delay on the “May Spray,” there is a good chance that there is enough residual control of the material. Sprays of DMIs and strobilurins provide protection of covered tissue for 14-20 days in conditions without rain. If a fungicide hasn’t been applied since the late April/early May, a spray should be considered due to the lack of leaf protection. Any new growth that occurred since the last spray will not have adequate fungicide protection. Tolerance of disease. My perspective in managing foliar diseases is to achieve the goal of having active leaves present on the tree in late October. In other words, there can be some disease present on the trees, but the tree has not defoliated and still has the ability to photosynthesize. Many people have a different threshold, which usually

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David Doll0 commentsApril 27, 2012Diseases

Considering an Almond Rust Spray?

There have been a lot of inquiries in regards to the decision of making a fungicide application for almond rust. Rightfully so, as last year was a bad Rust year, and this year’s weather conditions are very similar. With that in mind, I would encourage people to consider a rust spray this year, especially since: 1. The severity of the rust problem last year was widespread, therefore inoculum is abundant; 2. Weather conditions have been mild and humid, conditions that favor fungal foliar pathogens; 3. Most people have not applied a fungicide in several weeks. Rust timings for almonds begins at five weeks post petal fall and carries into the summer. Most fungicides are effective on rust. Of note are the strobilurins and DMIs. Avoid Copper, Iprodione, Topsin-M, Rally, and Laredo as they shown the least amount of control in the last round of efficacy trials conducted by Dr. Jim Adaskaveg.  Sulfur and Microthiol can be used, but multiple applications are recommended (I.e. Now, Late May/early June). Common missteps I see in regards to rust control using fungicides: 1. Trying to split the scab and rust treatment – applying a fungicide at one timing for control of both diseases. This spray is usually applied at five weeks post petal fall and usually misses one of the foliar pathogens; 2. Wanting a free ride for the fungicide. This means that they wait until the spray for mites. In cool years, mite sprays are delayed, and  therefore the rust application is delayed as well. 3. Not understanding that sprays need to be made before symptoms are observed. Upon orchard infection, it is very difficult, if not impossible, to control a rust epidemic; 4. Expectations of no disease. In bad years, the goal should be to keep the leaves on the trees until late October/early November. Having some rust present

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David Doll2 CommentsMarch 16, 2012Diseases

Scab Spray Timing and Other Spring Disease Questions

Wet weather has found its way into the valley, with rain events predicted off-and-on for the next week to 10 days. I have had a few questions regarding fungicide applications. Chlorothalonil (i.e. Bravo) isn’t registered for jacket rot of almond? Should I tank mix in another fungicide to provide protection? Figure 1: Overwintering scab lesions on the green wood of almonds. That is correct – Almond jacket rot is not on the label for chlorothalonil. Therefore, if a recommendation is written for jacket rot, another labeled fungicide must be used. FYI – chlorothalonil is labeled for jacket rot of apricot, which is caused by the same pathogens (Botrytis cinerea, Sclerotinia sclerotiorum, Monilinia laxa,Monilinia fructicola). What is the best time to apply a fungicide for Scab? Scab (Cladosporium carphophilum) overwinters on twigs of almonds (Figure 1) and will re-infect new leaves shortly after leaf out. Control of this diseases has been documented with dormant applications, petal fall applications, and applications made 2-5 weeks after petal fall. Earlier post-petal fall applications (~2 weeks) provides more effective control than later applications. Figure 2: Scab lesions sporulating on an almond branch. Note the darkened circles within the lesion. Fungicides should be applied for protection when this is observed to prevent scab infection. Photo courtesy of the Almond Board of California. The timing of the fungicide application should be made when the overwintering scab lesions begins to sporulate (release spores into the orchard).  When sporulating, the lesions, which are found on the green twigs of the tree, will develop a dark ring (Figure 2). Providing protection of the leaves with fungicides at this point will help reduce/prevent scab infection of the almond trees. Is there any way of determining if we are going to have another bad year of rust or scab? Predicting disease epidemics is challenging. Basic epidemiology indicates

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David Doll0 commentsMarch 2, 2012Horticulture

Varietal Bloom Issues

Weak bloom experienced in an orchard:Nonpareil and Monterrey shown. I have received more calls about weak bloom on almond pollinators. These trees have low counts of fruit bud, and the vegetative buds have pushed giving the tree a green instead of white appearance. The reports have been coming in from orchards of “juvenile orchards” (7-10 years of age), which typically have yielded quite heavy for their age. Nonpareil is affected, but the later harvested varieties, which include Monterrey and Fritz, are affected more severely.   In being in these orchards, it is clear that the period of fruit bud development was affected sometime last year. Fruit bud development/floral initiation varies among varieties. Studies have shown that ‘Nonpareil’s’ fruit bud differentiates about 7-14 days after harvest, while other varieties, such as ‘Carmel’, the majority of fruit buds differentiate prior to ‘Carmel’ harvest. This timing will be delayed in cool, mild years. Tree stressed imposed during this period will reduce fruit bud set, and the buds will remain vegetative (leaf buds). Very fruit bud are found on the trees,the majority of the buds are vegetative (leaf). In regards to the weak bloom, working with the farmers, we were able to identify some of the possible stressors that may have occurred during the period of fruit bud differentiation. Although we are unsure of the exact problem(s), the ones we are considering include:1. Holding back of irrigation in order to “force” hull-split or harvest,2. Excessive foliar disease, including rust or scab, that caused defoliation,3. Improper nutrient management – usually very low potassium and boron, and 4. heavy yields that reduced the vigor of the tree, impacting growth and the number of fruiting spurs available for future crops. In many cases, more than one of these issues could have affected the orchard. In orchards that experienced defoliation to foliar disease, many

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David Doll2 CommentsFebruary 15, 2012Diseases

Managing and Preventing Fungicide Resistance

The use of FRAC numbers was created to help determine the mode of action of the varying fungicides. One class of fungicide may have differing active ingredients and be sold under several different trade names. For example, FRAC group 3 is sold as Bumper, Tilt, Elite, Indar, Laredo, and Quash. Regardless of the trade name or active ingredient, any one of these fungicides within a FRAC group kills the fungal pathogen the same way. In order to prevent fungicide resistant pathogens, back-to-back applications of any one FRAC group should be avoided. Figure 1: Application of a fungicide to control a foliar pathogen. The blue dot represents a resistant fungal colony to fungicide A. The formation of fungicide resistance by many pathogens is inevitable as it is basically a game of odds. Making an application of one fungicide kills the majority of the fungi in the environment that are susceptible to that particular fungicide. There are always a few colonies of the targeted fungi that “escape,” and have some degree of resistance to the chemistry (lets say odds of 1/10,000) (Figure 1). This may seem surprising, but this is due to the varying genes found within the same species of fungi (It is no different why you catch a cold, and your coworker remains healthy). Making a second application of the same chemistry provides the environment for the “escaped” population to continue to reproduce and colonize healthy tissues (Figure 2). Eventually, the “escaped” population will become the majority of the population (Figure 3), and attempts to control the pathogen by spraying the same fungicide over and over will not occur (Figure 4A). Figure 2: After fungicide A application, the resistant blue colony “escapes” and begins colonizing other healthy tissues, increasing in population. Employing resistant strategies such as fungicide rotation can delay resistance formation. By avoiding back-to-back

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David Doll3 CommentsFebruary 9, 2012Diseases

Bloom Time Diseases and Control

Almond orchards are usually treated once or more during bloom for prevention of fungal diseases. These diseases include Brown Rot, Jacket Rot, Shot-hole, and Anthracnose. Flowers, flower parts, and young nuts are susceptible to infection, and often serve as the primary point infection for these diseases. Lack of properly timed sprays can provide the opportunity for a large amount of Brown rot infection of an almondblossom. The grey “fuzz” is the sporesproduced by the fungus.   infections. These epidemics create lingering disease issues which will cause orchard loss for the present and future cropping years. Brown rot, Monilinia laxa, infects the petals, pistils, and stamens of the almond flower. Upon infection, it moves into the woodier tissues of the tree, killing branches and forming cankers. All varieties are susceptible to brown rot, but they vary in their degree of susceptibility. Butte, Mission, and Wood Colony are the most susceptible; Carmel, Sonora, Fritz, and Monterey are moderately susceptible, while Aldrich, Nonpareil, and Peerless are the least susceptible. Brown Rot Blossom blight is usually controlled with a pink bud and full bloom treatments. Many of UCCE trials indicate that the full bloom treatment is the most important. A third petal fall spray may be necessary in years favorable to disease, especially if bloom is strung out and the weather is wet and rainy. A good rule to follow is that no more than ten days should elapse between treatments in wet weather. Canker caused by brown rot within Butte. This canker serves as a restingstructure for the fungus. A few growers have indicated that they do not spray for brown rot. This may be possible if good conditions persist through bloom AND the varieties planted are not very susceptible to infection (i.e. Nonpareil). It is important to note that not spraying can lead

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David Doll0 commentsAugust 26, 2011Diseases

Alternaria Found within Merced County

Relatively rare in the northern San Joaquin Valley, I was in an orchard today with Alternaria Leaf Spot. Caused by the pathogen Alternaria alternata, this disease usually appears by mid summer and causes defoliation. Significant problems with Alternaria have been observed in the southern San Joaquin Valley and northenr Sacramento Valley. In some of these orchards, resistance to strobilurins fungicides has occurred, which indicates the need to properly rotate fungicide chemistries when spraying to control this disease. Please see this UC Almond IPM page for more information. Here are some pictures to help you identify Alternaria leaf spot: Leaf symptoms of Alternaria on Carmel Another shot of Alternaria Leaf Spot on Carmel Leaf Lesion of Alternaria found on an almond leaf. Note the zonate appearance of the lesion and black specks – these are the identifying symptoms of this disease. Another photo showing the zonate appearance of the lesion.

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David Doll0 commentsAugust 20, 2011Diseases

Reports of Hull Rot

I have been getting a few calls regarding hull rot. I have covered this topic before (link to previous article), but some of the particular questions regarding the disease are answered below. What is Hull Rot?Hull rot is infection of the almond hull by two fungi, Rhizopus stolonifer (common bread mold), or Monilinia fructicola. These pathogens are common throughout the environment, and are, in this case, serving as opportunistic pathogens. Upon infection, they release toxins that are translocated into the fruiting wood, which kills the wood, causing crop loss. What do you mean by opportunistic pathogen?Once the hull splits, the perfect micro-climate for fungi is created. The hull is full of nutrients and water – the two things that fungi want from a host. Since the spores of these fungi are found throughout the air, they invade the newly split tissue, infecting, and completing their life cycle. By reducing the “home” for the fungi, we can reduce the number of hull rot strikes. These strategies include reducing the water and nutrient content of the hull. How can I prevent Hull Rot?Asking “How to prevent” is a good start. Prevention is the key to reducing hull rot strikes. Hull rot often affects high vigor orchards. Nitrogen should not be applied after kernel development is completed. This is typically the end of May, but this year it extended into mid-June. A slight to moderate water stress at the onset of hull-split should be applied. Applying both of these practices to the orchard has been shown in multiple research trials to reduce hull rot by 80-90%. Often times, this is all growers need to do to prevent severe hull rot issues. I tried those things, I still have hull rot!Severe outbreaks of hull rot have been observed. Last year, hull rot infections occurred in many orchards. I

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David Doll0 commentsAugust 12, 2011Diseases

Summer Foliar Disease Epidemics – Scab

Scab lesions found on almond twigs. I have scab all over my orchard. What can I do? Short answer – nothing. It is too late to expect control of the disease. Once the fungus infects the leaf and produces visible symptoms, that is a clear indication that the fungus has colonized the inside of the leaf. No fungicide completely penetrates the leaf surface, which makes it impossible to cure an infected leaf. Furthermore, spraying a fungicide upon seeing a disease epidemic will end up doing more harm than good. Since the fungus has completed it’s life cycle several times (Scab is a polycyclic disease), the population within the orchard is so high that making an application can lead to the selection of a fungicide resistant strain of scab. Scab lesions found on the underside of almond leaves. If every tree has scab, what can I expect? Defoliation. As the fungus colonizes the leaf, it begins producing spores which are the reproductive structures of the fungus. These spores are produced and move from an infected leaf to a healthy leaf. Upon complete colonization, the tree detects that the leaf is infected, forms an abscission layer which causes the leaf to fall from the tree. If enough leaves are infected on the tree, total defoliation can occur – and in most cases, is expected. Symptoms of scab observed on the top-side of almond leaves. What should I do? Since scab overwinters on the infected twigs in the form of lesions/fruiting bodies, population reduction is based upon controlling the disease present within the field, and coverage of susceptible tissues in the spring. Dormant applications of of copper and oil or chlorothalonil and oil have been shown to reduce summer populations of scab, and are recommended in orchards that have experienced leaf loss the previous summer/fall. Prophylactic sprays

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