David Doll0 commentsSeptember 14, 2016Diseases

Defoliating for Rust Management…Good Idea?

Written by David Doll (UCCE Merced County) and Brent Holtz (UCCE San Joaquin County) The foliar disease rust caused by the fungus Tranzschelia discolor has been a major problem this year. This disease is characterized by rusty-brown- or black-colored spores that form on the lower side and angular chlorotic (yellow) lesions on the upper side of the leaf. Rust can spread easily from orchard to orchard with wind, and minimal periods of leaf wetness are required for the pathogen to infect. This past year, long periods of conducive conditions, which include warmer temperatures, late spring rains, and increased canopy humidity were experienced, making rust management a challenge. Rust is easily controlled by properly timed fungicides. Several modes of action provide protection. Those with the highest activity are FRAC Groups 3, 11, and 19 or mixtures of these groups. Also several broad-spectrum fungicides such as micronized sulfur (M2, microthiol) and chlorothalonil (M5, Bravo) have very good efficacy with short- and long-residual activity, respectively. Since rust can infect from spring to summer, sprays need to be timed accordingly. Spring sprays (2-5 weeks after petal fall) based on monitoring for rust symptoms is often used to initiate rust control practices at the beginning of a potential epidemic. One fungicide application, however, may not provide effective control if favorable conditions persist and later sprays may be needed. Typically, spring-time disease management programs for scab and Alternaria leaf spot will also provide management for rust. High populations of rust can prematurely defoliate trees. As the fungus spreads, it ruptures leaf tissues, reducing the photosynthetic potential of the leaf, and eventually causes leaves to fall. If too many leaves fall from the tree, the tree will re-leaf, reducing the cropping potential for next year. Although this re-leafing is concerning, it is better for the tree to re-leaf

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David Doll0 commentsApril 6, 2015General

Mid-Spring Considerations/Update

The 2015 almond season is in full swing. Many orchards have went through their drop, fertilizer applications and irrigation have began, and pest management concerns are beginning to form. Below are some thoughts and considerations from recent field visits. 1. Crop development is ahead of schedule – by as much as a month. Kernel fill is beginning in many ‘Nonpareil’ orchards, meaning that ‘Butte’/’Padre’ orchards won’t be far behind. This timing is important as about 80% of the season’s nitrogen budget should be applied prior to kernel fill – which may occur as early as early May! Secondly, it indicates that at this point – “What you see is what you’ll maximally get” in regards to kernel size, and farming practices – more particularly irrigation practices – can only reduce crop size. 2. Peach Twig Borer’s (PTB) “May Spray” timing looks to be an April timing. In the Merced area, our traps picked up a biofix around March 15th, which means we will be approaching 300 DD in about 7-10 days. Keep in mind that the spring spray timing for PTB is 300-400 degree days after the biofix. It may also be possible for the May Spray to have some effect on NOW.  Frank Zalom (UC Davis Entomologist) has found through research trials that timing the spray to 100 DD post NOW biofix (egg traps) and around 400 DD post PTB biofix can provide good control of NOW and PTB at harvest. 3. Rain is predicted for Tuesday (the 7th). Hopefully some rain will fall in the valley to help with the multitude of water issues. This rain may also bring some potential for fungal disease – especially rust and scab. Consider an application if there has been a history of disease. This application can be made 1-2 days after the rain event and

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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 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 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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