David Doll2 CommentsNovember 16, 2010Diseases, Insects

Preparing for the Dormant Period

Just returned from a field day meeting held near Firebaugh discussing pest and disease management strategies for the upcoming dormant season. I thought I would highlight a few points from the talks by Walt Bentley (UC IPM), Mario Viveros (Emeritus UCCE Farm Advisor – Kern County), and myself – David Doll (UCCE Farm Advisor – Merced County). I apologize for the brevity of the entry as I am preparing for a field trial due to be fumigated next Monday. Navel Orange Worm (NOW): Winter sanitation is critical to help reduce the overwintering population of NOW. In general, no more than two mummies per tree should be left in the tree. With some growers, complete removal of the mummies is performed. Most growers remove mummies. This is easily done on younger trees. As the trees mature and become larger in size, the task of sanitation becomes difficult, resulting in more mummies remaining within the upper canopy. This may explain why many growers see more NOW damage in mature blocks. Even though the trees may appear to be clean, a closer look usually reveals mummies still hanging in the tree. Peach Twig Borer (PTB): Scouting for hibernacula should be performed. There are several treatment options and timings for PTB. These include dormant oil sprays, bloom sprays with reduced risk products, and May sprays. See the PTB page on the UC IPM website for more information. Mites: European Red Mite and Brown Almond Mite will overwinter as eggs around the base of spurs. Spur sampling, which should also be done for scale, can help determine if treatment is needed. An oil spray outlines at the Mite page on the UC IPM website will suffice for most locations. Scale: The dormant period is the easiest time to treat for scale. Spur sampling can help determine the treatments needed. Check here for

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David Doll0 commentsOctober 12, 2010Diseases

Tree Defoliation due to Rust or Scab

Due to the wet year, foliar diseases have been problematic. In many cases, growers have observed late season defoliation due to rust (Tranzschelia discolor f. sp. dulcis) and scab (Cladosporium carpophilum). These two foliar fungi initially infect the tree in mid to late spring, reproducing within the canopy of the orchard, killing leaves by late summer/early fall. The fungi must be controlled before the infection occurs to prevent late season defoliation. To help prevent an increase of rust and scab for the coming growing season, the following practices are recommended: Rust: Rust symptoms on almond leaves. Note the brown spores on the underside of the leaf. Orchards that have defoliation due to rust should have zinc sulfate (20-40 lbs/acre) applied in late October/early November. This practice will hasten leaf fall. Once the leaves are in contact with the soil, the leaves will begin to break down, reducing the overwintering inoculum. The rust fungus is not a soil inhabiting fungi and will not survive without leaf tissue. It is also important to remove any leaves from the crotch of the tree. These leaves serve as an overwintering reservoir and infect leaves the following spring. Fungicide sprays should be considered to help control another outbreak the following year. Orchards with a history of rust should apply sulfur or maneb 5 weeks after petal fall and follow 4 to 5 weeks later in late spring and summer with a FRAC Group number 11 fungicide (strobilurin) to control leaf infections. To be effective, fungicides must be applied before rust symptoms are visible. Scab: Scab lesions on almond twigs. These lesions are fruiting bodies that can re-infect the tree the following spring. Orchards experiencing defoliation due to scab should have a dormant application of either copper/oil or liquid-lime sulfur. These applications will help reduce and/or delay the

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David Doll2 CommentsJuly 6, 2010Diseases

Unusual Leaf Symptoms in 2010- Some thoughts…

Several calls have been coming in over the past few weeks regarding unusual leaf symptoms on almonds. For the most part, the symptoms have appeared across varieties, but with one variety being more heavily affected within a single orchard. Symptoms also have appeared in waves, starting about mid-April, again in May, and then again in June. Orchards all over the San Joaquin Valley have been calling in with reports, varying in ages, varieties, nutritional status, and orchard spray practices. The symptoms appear to start off as small, yellow pin-head sized lesions on the leaf (Figure 1). Following a few days of heat, the lesions enlarge, and become necrotic, often abscissing from the leaf (Figure 2). The margins of the leaf appear to be the most severely affected, with some damage evident throughout the leaf blade. Figure 1: Initial damage observed of foliar problem commonly experienced in the 2010 crop year. Photo provided by Allan James, Mid Valley Ag. Figure 2: Damage to almond leaves experienced in the spring of the 2010 growing season. These symptoms appeared later than the symptoms observed in Figure 1, usually after a hot spell. I initially thought that the problem was due to foliar nutrient sprays – either due to contaminated products or cation activation (iron, zinc, manganese, magnesium, etc.) by a rain event. This didnt seem to fit the symptoms in every orchard due to some orchard owners not spraying foliar nutrients. Symptoms also appeared similar to potassium deficiency, but the widespread occurrence throughout the valley does not support a nutrient deficiency. The possibility of corky spot was brought up, especially since the leaf symptoms look similar to the pictures of this disease. Corky spot is an unknown disorder that tends to be associated with nonpareil planted on seedling almond type rootstock (i.e. bitter almond). Due to the widespread occurrence of this symptom across differing orchard ages, nursery

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David Doll0 commentsApril 20, 2010Diseases

SIlver Leaf of Almond – Another heart-wood rotter

Silver leaf is a fungal disease of the xylem tissues of many various species of fruit trees. Caused by t he fungus Chondrostereum purpureum, it is commonly found in riparian areas containing hardwood trees such as willows, poplar, birch, and oaks. Figure 1: Padre leaves showing symptoms of silverleaf (left) versus leaves from a healthy Padre tree (right). A farm call this week brought me to an orchard showing signs of silver leaf infection. Silver leaf is so named because of its ability to make laves appear silver in color (Figures 1 and 2). This appearance is due to a toxin produced by the fungus that is carried through the xylem to the leaves. Once within the leaves, it creates a separation between the epidermis and palisade layer, creating an air gap that interferes with the normal interception of light. As the disease progresses, leaves curl upward at the edges and turn brown (Figure 3). Scaffolds will eventually collapse, shortly followed by tree death. Figure 2: Close up comparison of a Padre leaf showing symptoms of silverleaf (left) versus a healthy leaf (Right). Irregular cankers found within the xylem tissue of dead and/or dying branches is another characteristic symptom of the disease (Figure 4). In later stages of tree decline, spore-bearing conks or shelf like mushrooms form in the fall. Generally found on the north side, these mushrooms are gray to white in appearance and have a smooth, purplish lower surface. The spores from the mushrooms can infect neighboring trees, and are released upon rain events. Exited spores are moved by the wind and the mushrooms can produce spores for 2 years. Figure 3: Severe infections of silverleaf are characterized by curled leaves and necrotic margins. Wounds with exposed xylem or sapwood or vulnerable to infection. This includes pruning wound

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David Doll0 commentsApril 5, 2010Diseases

Almond Leaf Rust – Treat now to prevent late season defoliation

Almond leaf rust (Tranzschelia discolor f. sp. dulcis)can cause defoliation of almond trees during the late season. Rust is favored by high humidity and is worse in years in which late rains occur. Appearing as small yellow lesions on the upper surface of leaves with brownish/red pustules on the bottom (Pictures below), rust will first appear in late spring or early summer. The disease does not appear to affect the fruit. Spread through the orchard is through air movement, which blows spores from an infected leaf to an uninfected leaf. Infected leaves will eventually fall off of the tree affecting crop and tree health. The disease overwinters on leaf material. Prevention: Orchards favoring high humidity often have rust problems. Encouraging air movement by planting on wider spacings (22′ between rows), hedging, or selective pruning may help reduce canopy humidity. Microsprinklers and solid set sprinklers may increase canopy humidity since evaporation of sprayed water may occur. Sanitation (leaf mowing, breakdown) should be employed to reduce overwintering inoculum. Treatment: In orchards that have a history of rust, a two spray fungicide program should be used to reduce disease and clean up the orchard. The first application should be applied 5 weeks after petal fall and followed up with a 2nd application at 10 weeks post petal fall. DMI (FRAC Group 3) or strobilurins (FRAC Group 11) provide good to excellent control. Broad spectrum fungicides such as sulfur and topsin provide a good, cheap control and also allow an option for fungicide rotation. More information on fungicide efficacy can be found here. Applications of zinc sulfate (20-40 lbs/acre) applied in late October/early November should be made to help reduce overwintering populations of rust. The zinc will hasten leaf fall, and prevent the rust inoculum from increasing. In orchards of severe infestation, applying a

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David Doll4 CommentsMarch 29, 2010Diseases

More rain – should I spray?

Rain is forecasted for the coming week. Many growers are wondering “Should I apply another fungicide?” Foliar diseases of concern at this time of year are scab, shothole, and anthracnose. Conditions of leaf wetness and mild conditions favor growth of these fungi, which can infect and kill leaves, nuts, spurs, and shoots. Currently, the forecast calls for 2-3 days of rain. If it rains all three days, there would be ample time for these fungi to infect your trees. Post-rain event weather conditions should also be considered when making this decision. If the weather looks to be warm and breezy, a spray might not be necessary as the conditions will dry out the trees. Some level of disease may occur, but the rate of secondary spread would be reduced to improper environmental conditions. A rain event following 4-5 days later, however, would increase the disease pressure within the orchard. Since spring rain events tend to be one to two days in length, broad spectrum fungicides are a viable alternative for orchard applications. Applications of Captan, Captevate, Maneb, and/or Ziram can provide a cheaper treatment option than some of the other fungicides. These products provide moderate to good control of scab, shothole, and anthracnose and reduce the chance of resistance formation. For more information of treatment options and fungicide efficacy, please see the Fungicide Efficacy and Timing Charts for Almond.

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David Doll9 CommentsFebruary 23, 2010Diseases

Almond Anthracnose – An ugly spring disease

Anthracnose (Colletotrichum gloeosporioides) is a damaging fungal disease that attacks the blossoms, leaves, fruit, and limbs of an almond tree. Infected leaves tend to develop water-soaked lesions that eventually fade in color (Figure 1). Defoliation can occur, but leaves often remain attached to the branches. Infected nuts often have a crater-like lesion in which the affected area turns a reddish orange(Figure 2). Often, the fruit may gum profusely as the fungus is able to penetrate into the kernel, killing the embryo. Affected nuts often remained attached to the spur(Figure 3). Shoots and spurs that bear infected nuts often become infected and die(Figure 4). Lack of culturing the fungus from these branches suggests that the wood does not die by direct fungal colonization, but by a produced toxin. Thus, this disease affects the current year’s crop as well as affecting the future crop by damaging and weakening the affected scaffolds.Figure 1: Almond leaves infected by Anthracnose. Figure 2: Anthracnose infection of an almond. Note the profuse gumming coming from the kernal. Anthracnose is most severe in wet, warm springs (> 60F). Orchards that have a history of anthracnose should be treated during bloom (pink tip forward) to help protect blossoms. Repeated fungicide applications may be necessary through the spring as trees should be protected before rain events. Fungicides provide protection for up to 7-10 days, and rain events occurring beyond this “control window” may require another spray. All varieties are susceptible to Anthracnose, but they vary in their degree of susceptibility. Butte, Thompson, Merced, Price, Peerless, Winters, Monterey and Fritz are the most susceptible varieties. Harvey, Carmel, Ne Plus Ultra, Padre and Mission are moderately susceptible, with Nonpareil being the least susceptible. Figure 3: Kernals killed by anthracnose often remain on the tree as dried, shriveled mummies. Figure 4: Anthracnose infection

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David Doll2 CommentsFebruary 23, 2010Diseases

UPDATE: DMI Fungicides and Botrytis Control

Last week, I suggested the use of DMIs (Quash, Indar, Inspire, Rally, etc.) for brown rot control spray program. A fellow reader shared some experiences with me regarding the use of DMI fungicides during the late bloom period. Although DMIs are good for brown rot, anthracnose, and have some activity against scab, shothole and rust, they do not provide control for the fungus Botrytis, which causes jacket rot and green fruit rot (Photo above). Botrytis is found everywhere and grows rapidly in cool, moist weather. It typically infects young almond jacket and fruit, causing infection and nut abortion. It is greyish in appearance, and affects varieties that cluster – Avalon, for example. It is not found frequently, but when found, is typically in orchards that have only used DMI fungicides. Therefore, if your last fungicide application is a DMI, I would advise to include a broad spectrum as a tank mix (chlorothalonil, Captan, Maneb, Rovral) to help reduce the incidence of Botrytis infection. Any other thoughts on this?

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David Doll1 CommentFebruary 15, 2010Diseases

Brown Rot: A disease problem during bloom

Almond orchards are treated at least once during bloom for fungal disease brown rot. Rightfully so, as the brown rot fungi (Monilinia laxa) can kill the flower, fruiting spurs, and shoots. Flowers are susceptible from pink bud to petal fall, and are most susceptible when flowers are fully open. The fungus is able to infect all parts of the flower: anthers, pistils, petals, and stigmas (Figure 1). Upon killing the flower, it can move into and kill woodier tissues of the tree. Once in the woodier tissue, it forms a canker (Figure 2). This canker can enlarge to kill the branch and also serves as the survival structure for the fungus. These cankers are the reason why it takes many years to “clean up” an orchard after a severe brown rot infection. Brown rot can also infect the jacket of the almond and kill the young fruit as well. This is called jacket rot. Figure 1: Blossom blight of almond caused by the brown rot fungus Monilinia laxa.All varieties are susceptible to brown rot, but they vary in their degree of susceptibility. Butte is the most susceptible variety followed by Carmel. Ne Plus Ultra and Mission are moderately susceptible, while Nonpareil and Peerless are the least susceptible to brown rot. Brown Rot Blossom blight is usually controlled with a pink bud and full bloom treatment. Many of Brent Holtz’s (UCCE San Joaquin) trials suggest that the full bloom treatment may be the most important. A third petal fall spray may be necessary in years favorable to disease (rain). If bloom is strung out and the weather is wet and rainy, no more than ten days should elapse between treatments. A few growers have told me that they do not spray for brown rot. This may be okay if good conditions

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Brent Holtz0 commentsFebruary 2, 2010Diseases

Bloom sprays: What and when to use to provide effective disease control.

Written by Brent Holtz, UCCE San Joaquin Almond trees are susceptible to bloom and foliar diseases when it rains at bloom, and the decision of when to spray and what fungicides to use can be quite difficult. In the San Joaquin Valley we are usually in a low precipitation region and we cannot predict when and how much it is going to rain. We often receive rain during bloom which can result in favorable conditions for several plant pathogenic fungi to cause spring time diseases of almonds. The main diseases in almonds are Brown Rot Blossom Blight, Green Fruit Rot or Jacket Rot, and Shothole. Other less familiar diseases include Scab, Rust, Leaf Blight and Anthracnose. The fungi that cause these diseases are usually always present in almond orchards, sometimes in higher or lower amounts depending on the previous year’s disease levels and current environmental conditions. Fungicide Control ProgramsGenerally, a good disease control 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 on all diseases (fig. 1). It is a good idea to use more than one kind of fungicide for a broader spectrum of activity. This will be especially important with using Strobilurin fungicides (Abound, Cabrio, Flint, Sovran, and Pristine). Pristine is a Strobilurin fungicide that is combined with Carboxyanilide to slow resistance development. Resistance to these fungicides can develop over time and repeated use, thus try to rotate the fungicides you use. Two successive applications of the same mode of action should be avoided. Information on effective fungicide rotations can be found in figure 2. A suggested bloom spray strategy:Usually two sprays are made for brown rot control. The first is usually done at

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