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 Doll0 commentsMay 14, 2011Diseases

Rain this weekend?

Although a late entry for the week, I figured I better address an issue that is facing many almond growers: a late spring rain storm. A rain event is predicted for Sunday with possible carry-over to Monday. I have received a few calls regarding the decision to spray fungicides for various foliar diseases. In many cases, making an application for a one day rain event is not warranted unless there is a persistent disease problem. Spray applications should be considered if there is a history of almond anthracnose, or in orchards with severe brown rot infestations. If sprays are made, try to use broad spectrum fungicides that target multiple pathways. Using these products will help reduce the formation of resistance. If you do not have a history of these diseases, I would hold off on making an application as the winds and warm weather will usually dry the almond leaves before a severe outbreak occurs. If rust is a concern, Dr. Adaskaveg from UC Riverside has shown that control of almond rust can be achieved even after initial symptom development. He has recommended sprays of sulfur or strobilurins and encourages growers to scout their orchards for the presence of the disease. This is great news since it provides growers with the ability to treat upon seeing an infection versus a prophylactic spray for a disease that may not occur. Keep this in mind as we enter late May and early June – rust symptoms begin to appear in these months.

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David Doll4 CommentsApril 20, 2011Diseases

Phytophthora syringae and First leaf Tree Infections

Figure 1: Young almond tree killed by Phytophthora syringae. This year, Phytophthora syringae infections have been quite common. This is most likely due to the long, cool wet spring. Many new orchards have seen 3% or greater infection rate, especially in the variety Nonpareil. Figure 1 shows symptomatology of a tree affected by P. syringae. Leaves on these trees are lesion free, but tend to dry up, becoming crispy. Trees are not always killed, but they may be stunted in comparison to other trees within the orchard.  Examination of the trunk will typically yield amber “gumballs,” which often mark the edge of the fungus canker (Figure 2). When the bark is removed, the canker becomes visible (Figure 3). This disease has been observed frequently, and is commonly referred to as “bundle rot.” Figure 2: Amber colored “gumballs” caused by tree infection with Phytophthora syringae. Although common this year,  infection by P. syringae does not typically cause such a large problem. When temperatures warm above 75F, the fungus within the canker dies, and the affected area heals over as the tree resumes normal growth. Weather this year has been very mild, with few days above 75F. The fungus infects the trees through wounds that occur from digging, processing, delivering, planting, and rough weather. Upon infection, the fungus may remain latent until the tree breaks dormancy. If conditions remain favorable, the fungus continues to grow, eventually killing the tree. Most likely, we do not have a higher amount of this fungus within the orchard in comparison to any other year, but rather that the current cool and wet environmental conditions are promoting fungal growth, causing an increase in observed incidence. Figure 3: Bark removal shows fungal canker of an almond tree affected by Phytophthora syringae. If tree loss or damage occurs within your block due to P. syringae. It is better

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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 Doll0 commentsApril 6, 2011Diseases

Late Scab Timings for Almonds

Early season Scab lesions on almond. We are approaching the end of the effective almond scab control spray timings. Scab has become more common in area orchards due to the increased planting density and occurrence of wet springs. Causing defoliation in late summer, this disease can weaken the tree, reducing crop for the coming years. A heavily infected orchard will have persistent scab issues for several years after the disease flare up. Fritz and Carmel are more susceptible to this disease than Nonpareil and other varieties This spray period starts at about 2 weeks after petal fall and continues to about 5 weeks after petal fall. Years of research by Dr. Jim Adaskaveg and Dr. Brent Holtz have demonstrated that spraying during this period provides the greatest control. The use of strobilurins (FRAC 11 – Gem, Abound, Pristine) provides the greatest control of this disease, followed by the DMIs (FRAC 3 – Quash, Inspire, Inspire Super, Bumper, Tilt), Topsin-M (FRAC 1), and broad spectrums (Captan, Maneb, Ziram, etc). Overwintering scab twig lesions. It is speculated that strobilurin resistant populations of scab are becoming more common. Applying back-to-back applications of strobilurins will hasten the development of resistance. If you sprayed a strobilurin earlier this bloom season you must consider another mode of action for scab control. Earlier entries emphasized the importance of saving this mode of action for summer diseases.  Sprays must be made within the 2 – 5 week petal fall period to provide maximum control. Later sprays will not be as effective. Waiting till the disease is visible before you start to spray is not advise as the population at that point is too high to control. In many cases, spraying at this point will only waste money AND increase the risk of fungicide resistance.  Orchards with severe outbreaks should consider a dormant application of copper/oil to

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David Doll0 commentsMarch 8, 2011Diseases, Nutrient

Some Early Season Thoughts

I have been in a few orchards this past week and thought I would relay on some thoughts.1). Bloom. In comparison to last year, bloom seemed to last much longer. Some orchards I have been working next to are now entering petal fall. Their first blossoms opened up about 20 days ago. It appears that the trees hit full bloom in our area during the nice weather that we had last week. Hopefully this will aid in pollination and nut set. 2). Frost Damage.  Frost damage appeared to be pretty minimal (1-5%) in the 15 or so orchards I checked around Merced County. I have heard mumblings that damaged pistils and stamen from the extreme temperature may lead to poor pollination and nut set. At this time, it is hard to predict if this occurred and to what extent. Most likely this type of damage will become apparent during the flower drop and nut drop periods. Keep in mind that other weather issues may contribute to these drops making it hard to determine what caused what. 3). Fungicide Sprays. The threat of Brown Rot is still pretty high, especially with Butte/Padre orchards. This weekend we saw great weather for Brown Rot – over 60F and raining. The winds today have aided in the reduction of leaf wetness, thus reducing brown rot risk. If Sat/Sun/Monday’s rain event fell in the 10 day window from your last spray, it looks like you may be able to get through this week without too much worry – unless the weather changes! You have at least 10-12 days of protection after a fungicide spray. As petal fall approaches and we move into leaf out and nutlet formation, keep an eye out for shot-hole, jacket rot, anthracnose, and scab. Of these four diseases, scab requires the latest treatment

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Roger Duncan2 CommentsNovember 30, 2010Horticulture

Rootstocks for California Almond Orchards

Roger Duncan, University of California Cooperative Extension in Stanislaus County Joe Connell, UC Cooperative Extension in Butte County John Edstrom, UC Cooperative Extension in Colusa County Nemaguard has been the main rootstock of the California almond industry for many decades.  It is well adapted to the well-drained, non-calcareous loam and sandy loam soils common throughout the San Joaquin Valley.  Nemaguard is vigorous, immune to rootknot nematode, compatible with all almond varieties, doesn’t sucker much, has decent anchorage, and is easy to grow at the nursery.  Never-the-less, there are many situations in which nemaguard struggles.  Nemaguard, as with most peach rootstocks, is prone to lime induced chlorosis (yellowing / iron deficiency) in high pH soils.   It is also susceptible to salt toxicity (sodium, chloride & boron), “wet feet”, Phytophthora root rot, oak root fungus, crown gall and “heart rot”.  Despite the name “nemaguard”, this rootstock is susceptible to ring and root lesion nematodes.  The susceptibility to ring nematode leads to the danger of bacterial canker in orchards replanted into sandy soil.  Since 1998, UC Cooperative Extension farm advisors in Kern, Stanislaus, San Joaquin, Colusa and Butte Counties have researched several alternative rootstocks for California almond growers.  These include most of the common, commercially available rootstocks in California along with many newly available rootstocks from other countries.  While there is no such thing as the perfect rootstock, there are better choices than nemaguard where many chemical or physical soil problems exist.  Below are some suggestions for specific challenging situations. Heavy soil / poor drainage.  Nemaguard is not adapted to poorly drained soils and can become yellow, stunted or even die from lack of oxygen.  Lovell, another peach seedling rootstock, is not much better.  As a group, plum rootstocks are pretty tolerant to heavy soil.  Marianna 2624 has traditionally been the rootstock of

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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 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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