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 Doll2 CommentsApril 25, 2011Diseases, Weeds

Herbicide Drift or Fungal Disease?

Figure 1: What caused this damage: Herbicide drift or fungal disease? Leaf samples often appear on my desk with notes asking to identify the problem. More often than not, symptoms appear shothole-ish in nature, and growers are questioning their fungicide program (Figure 1). These symptoms, however, also appear from drifting herbicides. How can one tell the difference between three common herbicides and the fungal disease shot hole? Answer: look for fungal fruiting bodies. Shot hole infected leaves can be determined by the black fruiting body that is centrally located in the lesion on the top side of the leaf (Figure 2). Figure 2: Shot Hole affectedalmond leaf. Note the blackfruiting body in the centerof the lesions.  What about telling the difference between the three herbicides? This one is a little bit harder, but it comes down to the coloring of the leaf tissue surrounding the lesion. Kurt Hembree, Weed Management Farm Advisor in Fresno County and Brad Hanson, UC Weed Specialist, have noted the following:  lesions caused by paraquat are localized, tan in color, and remain attached (figure 3). Carfentrazone (Shark) lesions have halos that lack color (figure 4). Oxfluorfen (Goal) lesions tend to have a defined halo, sometimes purplish and/or yellow in color (figure 5).  Both of these herbicides will cause lesions to fall from the leaf. Figure 3: Shot Hole like damage caused by  Gramoxone (Paraquat). Note that the lesions do not fall from the leaf To help with the diagnosis, observe the location of the damage. Is it over the whole tree, or just lower few branches? Is it just on the outer rows? Is it across both varieties? What is the history of herbicide use? Herbicide damage tends to be found on the lower portion of the tree, with damage on all varieties within the orchard. Drift damage also tends to

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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 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 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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David Doll0 commentsFebruary 15, 2011Diseases

Some more bloom time thoughts…

Yesterday, I had a nice chat with Dr. Jim Adaskaveg, UC Specialist in Plant Pathology. As many of you may know, Dr. Adaskaveg conducts many of our foliar and bloom disease trials. Here were some additional comments to be added about bloom spray timing and last week’s post. Dr. Adaskaveg highlighted that all parts of the flower are susceptible, but for the most part the flower petal is the most susceptible and is the primary point of flower infection. Many of the fungicides used have what Dr. Adaskaveg termed “Reach Back Potential.” This basically means that a fungicide application controls the growth of infections that may have occurred BEFORE the fungicide application. In these cases, the fungicide does not serve as a curative, but rather as a fungal-static agent, or, in other words,  it slows the growth of the fungus but does not kill it. This provides more time for fertilization and petal abscission to occur, thus reducing brown rot incidence. He also pointed out that Brown Rot is most active in temperatures above 57 degrees F. It is important to note that infection can still occur below 57F, but the rate of growth is reduced. He suggested that growers with brown rot tolerant varieties (i.e. Nonpareil) and no history of brown rot may be able to get by with a minimal spray program. In some cases he observed 50% Nonpareil orchards in which the grower did not spray the Nonpareil; fungicides were applied only to the pollinators. This practice was not recommended for Butte plantings. Butte is very susceptible to brown rot. In regards to the upcoming weather and the beginning of bloom, he suggested holding the first spray until after the rains, trying to hit the 30% bloom mark on Nonpareil.This should provide coverage for the emerged Sonora blossoms. He felt comfortable with

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