David Doll4 CommentsFebruary 9, 2020Diseases, Horticulture

2020 Bloom Considerations

As the 2020 almond bloom/crop year begins, there are several considerations to keep in mind. These points may be useful to help understand observations from the field as well as provide an understanding of any direction received. Nut set. Flower set percentages will vary from year-to-year. For mature trees, about 20-30% of the flowers will set while younger trees will be slightly higher. Even though this is low, crop loads are still high due to the number of flowers that are in each tree. A mature tree will have an estimated 35,000-50,000 flowers, depending on tree size. For more information on this, please see this article: https://thealmonddoctor.com/2016/03/07/almond-set-and-nut-drop/ which discusses the results of a tagging study which followed the development of almonds through the season. External factors greatly influence the set percentages. Crop load from the previous year impacts the amount of energy available for flower formation and bloom. High yields from the year before will reduce the number of flowers that will set nuts. Poor weather conditions can reduce bee flight hours, kill flowers in freezing temperatures, or promote disease. The period of weather risk extends into the fertilization period, which continues for several weeks. Keep in mind that the crop is susceptible to frost and diseases as long as there is a risk of frost and rainfall.   Several factors can be mediated by good farming practices. These include the presence and strength of pollinators (usually honeybees), post-harvest practices, and nutrient deficiencies. Honeybees are required for varieties needing cross pollination (most CA orchards – 2.0-2.5 hives/acre) and recommended for self-compatible varieties (0.5-1.0 hive/acre, although the exact number is unclear). Stronger hives (8+ filled frames) are essentially an insurance policy for poor weather conditions. Hive numbers and placements should take into account the weather. Post-harvest treatment of the trees is

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David Doll2 CommentsMarch 30, 2018General, Horticulture

2018 Spring Update

Spring conditions over the past week have finally turned favorable for growth. After a very warm winter, a long bloom with periods of cold weather, followed by a few weeks of wet weather, trees are starting to grow and nuts are beginning to size. These weather conditions have led to a few interesting observations and considerations for the spring. Leaf lesions caused by bacteria. Observations from several fields have yielded the occurrence of bacterial leaf spots. These spots are thought to be caused by Pseudomonas syringae, the same bacteria that causes blossom blast. Lesions are often varied in shape and have a distinctive yellow halo. Generally, the tree recovers after shedding a few leaves. Conditions can worsen if rains persist. Copper treatments to prevent may do more harm than good due to tree sensitivity. Lesions can be easily confused with herbicide drift or shot-hole; review application history to determine if these could be potential causes. Vegetative bud failure. Just as we observed in 2015, we have been observing vegetative buds within last year’s growth failing to push. This “bud failure” is not the same as non-infectious bud failure (crazy top) as future generations of buds will be healthy. Although we are unsure of exactly why this type of bud failure occurs, we believe it to be due to a reduction of localized carbohydrates from the December and January temperature ranges. Explained simply, warm winter days and nights below freezing increase the rate of respiration, leading to reduced carbohydrates to assist with bud break. If levels drop too low, buds can die. Often, healthy buds below the affected area will grow and hide the damage that occurred. Sometimes, buds will break later than normal. More rain. Mid-range forecasts are predicting rain for the end of the first week of April. These

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David Doll7 CommentsMarch 17, 2017Diseases

Bacterial Canker and Blast

Over the past few weeks, there have been a high frequency of problems reported with bacterial canker and blast. Bacterial blast results in blighted blossoms and causes crop loss. Interestingly, when there is a high frequency of blast, the very similar disease of bacterial canker appears a few weeks later in young trees(we are starting to get reports of canker now). Both of these problems are complicated. They involve the bacterium Pseudomonas syringae, plant stress which is often mediated by nematodes, and the weather. P. syringae is an interesting bacteria. It lives naturally on plant surfaces as an epiphyte – meaning that it generally does not cause problems. During wet winters which create a favorable environment, bacterial populations increase and spread through the tree and orchard. At some point the population becomes large enough that it begins to invade plant tissues – starting with buds (bud drop), then flowers (blast), and finally bud scars and other openings which leads to bacterial canker. Tree health has a lot to do with the susceptibility to P. syringae infection. Generally, if the tree is healthy, infections rarely occur. Weak, young trees- usually three years or less- are susceptible to bacterial canker which can cause the loss of tree scaffolds or death. Mature orchards usually don’t exhibit bacterial canker symptoms, but rather have blast and bud drop problems. Orchards with severe problems are typically planted in sandy soils, which often tend to be acidic, hold less water and nutrients as well as being more conducive to plant parasitic nematodes. All of these factors, if managed improperly, can increase tree stress, increasing susceptibility. Some micronutrient deficiencies, notably iron, have been found to increase susceptibility, while balanced, proper major nutrient fertilization has been shown to reduce occurrence.  Severe water stress can also increase occurrence, and is thought to be due

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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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