David Doll0 commentsAugust 5, 2011Diseases

Summer Foliar Disease Epidemics – Rust

I have received quite a few calls regarding rust and scab. Over the next few weeks, I am going to answer the commonly asked questions about these diseases and provide a management strategy to help reduce disease pressure for future years.In regards to a general overview of Rust, please see the following:Almond Leaf Rust – The Almond Doctor from April 10th, 2010,Almond Rust – UCIPM Website. I have rust 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 (Rust 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 rust. If every tree has rust, 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. What should I do?Since rust overwinters on the infected leaves in the form of teliospores (black spores found on the bottom of the leaves), it is important to destroy/remove all leaf

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David Doll10 CommentsApril 7, 2011Diseases

Brown Rot or Bacterial Blast?

I have had a few calls regarding blossom dieback within almond trees. In many orchards, especially in Northern Merced, I have seen dried, crispy blossoms still stuck to the tree. In some cases, this may be brown rot, but in a year where we had a cold snap during bloom, this could also be bacterial blast. How do we tell the difference? Figure 1: Brown rot blight of an almond blossom. Note the grayish brown fuzz found at the base of the blossom. Brown rot will kill blossoms and will often move into the spurs and branches on the tree. As temperatures warm up, cankers form on the wood which sometimes cause gumming. These cankers serve as an over-season resting place for the fungus. Even more obvious than cankers is the formation of light brown to gray fuzz on the jacket or at the base of the flower (Figure 1). This “fuzz” is the spores of the fungus. Bacterial blast is a blossom blight that is caused by the bacteria Pseudomonas syringe. This is the same bacteria that causes bacterial canker and bud drop. Found naturally all over the tree’s surface, this bacteria is an opportunistic pathogen, meaning it will not affect a healthy tree. When a tree is stressed from nematodes (particularly ring nematode), poor soil conditions, lack of nutrients, or an extreme frost event, this bacteria makes it way into the tree’s tissues through cracks and natural openings killing affected tissues (Figure 2). Figure 2: Bacterial Blast of almond blossoms. Photo courtesy of M. Moran.  To determine if the dead blossoms in your almond trees was from blast or brown rot, ask yourself these questions:1). Did I spray for brown rot this year?2). Am I seeing the dead blossoms across all my varieties?3). Are the trees stressed in some way

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David Doll3 CommentsMarch 16, 2011Diseases

How long between fungicide applications?

I have been receiving a few questions about fungicide sprays over the past few days. i thought the answer may be helpful for others. “What fungi do I have to worry about this time of year?”Shot-hole, Anthracnose, Brown Rot, and Jacket Rot. For the varieties in petal fall/post petal fall, Anthracnose and Jacket Rot are the big ones to keep in mind. Most fungicides provide control for shot-hole, but not all provide control for Jacket Rot and Anthracnose. Make sure the spray you are spraying targets those pathogens. Remember, DMIs (FRAC 3) do not provide sufficient control of Botrytis, one of the pathogens that cause Jacket Rot. “How many days can I wait before I make another fungicide spray?”For the most part, fungicide sprays will last between 10 and 14 days, depending upon weather. If rainy conditions occur, the residual activity will be on the shorter side, while dry, warm, sunny weather may even extend the window beyond 14 days.. If continual wet, mild conditions that favor fungal growth persist, spraying every 12-14 days will provide control of most problematic leaf and flower infecting fungi.  “What if it rains, then clears up?”If post rain conditions are effective in reducing the duration of leaf wetness, a spray may not be needed after a single day rain event. Sunny, warm, and windy weather quickly dries the leaf surface, reducing the growth of shot hole, anthracnose, jacket rot, and brown rot. A single day of rain that falls on the later end of the spray window – lets say day 12 of the 14 day window – and has these post rain conditions probably will not warrant another spray. Several days of rain – 2-3days –  falling at the end of this period would. “When should I spray for Scab?”Scab sprays will should begin no earlier than

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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 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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