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It's Not the Right Time
The calls have come in. Citrus Dry Root Rot has struck. It's not supposed to happen this time of year. We've gone from this beautiful, rainy December to a mild winter mid-80s on some days. But, we had a fierce Santa Ana mid-January that knocked down street trees and caused other damage. And that's the cause. The devil wind. It's shocking how fast the citrus trees go from green to a lifeless brown.
BOOM!
The parting words of my predecessor Nick Sakovich (happily living in Hawaii) were: DRR hits trees between 1-15 years of age, less-well managed trees, trees that have been injured (gophers, weed whips, discing, fertilizer burn, etc.), in the presence of the Fusarium fungus, and it always happens on the first hot day of spring. A nice mild, or even cool winter, and then there is a sudden water demand by the tree on a hot day and BOOM, the tree's leaves turn brown in a weekend.
Dry Root Rot has menaced growers in Ventura County for many years. In the ‘50's and ‘60's it seemed most prevalent on older orange trees. A few years after the wet winter of 1968-69, dry root rot became an increasing problem among citrus trees of all ages. At that time, most of the damaged trees were on sweet rootstock (susceptible to Phytophthora), and growing in fine-textured soils or soils with poor drainage. A few years after another wet winter/spring (of 1983), dry root rot again reared its ugly head, but this time predominately on young lemons.
The disease is caused by the fungus, Fusarium solani. Fusarium in citrus clogs the xylem which carries water to the leaves. When damage is done to the xylem, the ability to carry water is reduced. When a lot of damage is done, that's when you get that BOOM.
The damage occurs at the base of the trunk. The bark has been cut away to show the affected wood beneath
This fungus is most likely present in all citrus soils in California. It is a weak pathogen in that by itself it will not attack a healthy tree. However, experiments conducted in the early 1980's by Dr. Gary Bender, showed that when seedlings were girdled, root invasion occurred. In the field, the fungus can infect trees once gophers have girdled the roots or crown. A Phytophthora infection will also predispose trees to Fusarium, as will asphyxiation. Therefore, the mere presence of the fungus in the orchard soil will not lead to the disease.
So we call it a disease, but it's really a complex of circumstances that need to be present. It has to be a wound, Fusarium and the weather. We forget that wind is a greater drain on a tree's water needs than heat is actually. Same for humans, you got out on a windy day, and you come home and the first thing you do is drink some water. That's the way food is freeze-dried.
And that's the stress we had this year, and BOOM!
There's not a whole lot that can be done after this happens. The way it is managed is through avoiding wounds and making sure that when stressful conditions of wind and heat are forecast, that the tree is well supplied with water.
Nick was mainly right. Dry Root Rot mainly happens with the first hot day of spring, but it can happen anytime a wounded, infected tree goes under water stress. And it can happen to older trees, too.
There's more to this story. Read more about this infuriating disease complex at the UC Integrated Pest Management website - https://www2.ipm.ucanr.edu/agriculture/citrus/Dry-Root-Rot/
And listen to Dr. Akif Eskalen's Dry Root Rot story at "What Are the UC Ag Experts Talking About?"
https://www.youtube.com/watch?v=K2fyBcC1HXk
Bam - Dry Root Rot Season
The calls have come in. We've gone from cool to hot and Dry Root Rot of Lemon has struck, It's shocking how fast the trees go down.
Dry Root Rot has menaced growers in Ventura County for many years. In the ‘50's and ‘60's it seemed most prevalent on older orange trees. A few years after the wet winter of 1968-69, dry root rot became an increasing problem among citrus trees of all ages. At that time, most of the damaged trees were on sweet rootstock (susceptible to Phytophthora), and growing in fine-textured soils or soils with poor drainage. A few years after another wet winter/spring (of 1983), dry root rot again reared its ugly head, but this time predominately on young lemons.
The disease is caused by the fungus, Fusarium solani. This fungus is most likely present in all citrus soils in California. It is a weak pathogen in that by itself it will not attack a healthy tree. However, experiments conducted in the early 1980's by Dr. Gary Bender, showed that when seedlings were girdled, root invasion occurred. In the field, the fungus can infect trees once gophers have girdled the roots or crown. A Phytophthora infection will also predispose trees to Fusarium, as will asphyxiation. Therefore, the mere presence of the fungus in the orchard soil will not lead to the disease.
Description
Fusarium is a soil borne fungus that invades the root system. Once infected, the entire root will turn reddish-purple to grayish-black. This is in contrast to a Phytophthora infection which, in many cases, will attack only the feeder roots, but when larger roots are infected, only the inner bark is decayed and it does not discolor the wood. In addition, when observing the cross section of a dry root rot infected trunk, a grayishbrown discoloration in the wood tissue can be observed.
Dry root rot is a root disease, but symptoms of the root decline are seen above ground. They are similar to any of the root and crown disorders such as Phytophthora root rot, oak root rot fungus (Armillaria) and gophers. The trees lack vigor, leaves begin to turn yellow and eventually drop (especially in hot weather) causing twig dieback. Finally, the foliage will become so sparse that one will be able to see through the canopy of the tree. A period of two to three years may pass from the time of invasion until noticeable wilt. Many times, the tree will collapse in the summer, after a period of prolonged heat. In the case of dry root rot, the collapse is so rapid that the tree dies with all the leaves still on the tree. When looking for symptoms of dry root rot, keep an eye out for symptoms of other maladies as well — Phytophthora, oak root rot fungus and gophers being the most prevalent.
As mentioned previously, in order for Fusarium to infect a tree, there must be a predisposing factor such as girdling from gopher feeding. However, since many trees collapse from dry root rot without any apparent predisposing factor, there are obviously other factors which we have yet to identify. Therefore, in 1998, a grower survey was developed, along with intensive soil and leaf sampling, to attempt to identify as many new predisposing factors as possible. They might be elements in the soil, either deficiencies or excesses, or specific cultural practices such as irrigation patterns or fertilizer practices. Twenty orchards were identified from which 20 soil and 20 leaf samples were taken in diseased areas and another 20 soil and 20 leaf samples were taken from adjacent healthy areas. The owners or managers of the properties were given a questionnaire to complete regarding a variety of cultural operations. The objective was to identify those factors that would correlate well to trees becoming infected with dry root rot.
Survey Results
Soil analysis - The following laboratory procedures were conducted to see if there was any correlation between the disease and either deficiencies or toxicities of these elements or
conditions: sodium, boron, salt level, pH and soil type (sand, loam, clay). For these elements or conditions, no correlation was found. It would appear that for our sampling sites, these conditions, whether favorable or not (toxic or deficient), did not play a major role in predisposing the tree to dry root rot.
Leaf analysis - The following elements were analyzed for their concentration within the leaf: nitrogen, potassium, phosphate, manganese, magnesium and zinc. Of these, three correlations were found. Zinc and manganese levels were substantially higher in diseased trees. The third correlation showed a potassium deficiency in diseased trees. However, we do not believe that dry root rot is caused by elevated levels of zinc or manganese, or by potassium deficiency, but rather are a result of the disease. Unfortunately, it seems that we have still not identified any elements in leaf analysis that truly correlates and points to a predisposing factor for disease development.
Control Measures – What Works and What Does Not
Early experiments conducted by Menge, Ohr and Sakovich showed that the following circumstances or operations do not influence the incidence of this disease: fungicidal treatments, wounding the tap root at time of planting, sandy versus clay textured soils, spring versus fall planting and soil mounding.
- In choosing your nursery tree, the choice of rootstock is not important in that, as far as we know, all rootstocks are susceptible to this disease. However, since Phytophthora is a major component in dry root rot development, choosing a rootstock like sweet orange would certainly put those trees in a high risk category. We recommend that growers use Phytophthora resistant rootstocks like C35 or Citrumelo.
Phytophthora. Publications written in the 1970's, and again noted by our survey, showed that Phytophthora is a major culprit in the dry root rot complex. To control dry root rot, it is essential that the Phytophthora, when present, be controlled. This can be accomplished by fungicidal treatments, and by the proper application and timing of irrigation water. Overwatering creates a favorable environment for the multiplication of the Phytophthora fungus.
Gophers. It is well known that gopher damage provides entry points for Fusarium. Controlling gophers is an important factor in reducing the potential of infection by Fusarium.
Control
We presently have no direct control for dry root rot. To control the disease, we must control the predisposing factors such as gophers, Phytophthora, poor drainage and over-watering. If the predisposing factor(s) cannot be identified for a given diseased orchard, it will indeed be difficult to control the disease. Two things are certain though: 1.) There are no chemicals to date which will control this disease; and 2.) Presently, there are no rootstocks resistant to the disease.
Listen to Akif Eskalen tell the Dry Root Rot story
https://www.youtube.com/watch?v=K2fyBcC1HXk&feature=youtu.be
dry root rot time
Citrus Dry Root Rot
Nick Sakovich, Emeritus Farm Advisor
Dry Root Rot has menaced growers in Ventura County for many years. In the ‘50's and ‘60's it seemed most prevalent on older orange trees. A few years after the wet winter of 1968-69, dry root rot became an increasing problem among citrus trees of all ages. At that time, most of the damaged trees were on sweet rootstock (susceptible to Phytophthora), and growing in fine-textured soils or soils with poor drainage. A few years after another wet winter/spring (of 1983), dry root rot again reared its ugly head, but this time predominately on young lemons.
The disease is caused by the fungus, Fusarium solani. This fungus is most likely present in all citrus soils in California. It is a weak pathogen in that by itself it will not attack a healthy tree. However, experiments conducted in the early 1980's by Dr. Gary Bender, showed that when seedlings were girdled, root invasion occurred. In the field, the fungus can infect trees once gophers have girdled the roots or crown. A Phytophthora infection will also predispose trees to Fusarium, as will asphyxiation. Therefore, the mere presence of the fungus in the orchard soil will not lead to the disease.
Description
Fusarium is a soil borne fungus that invades the root system. Once infected, the entire root will turn reddish-purple to grayish-black. This is in contrast to a Phytophthora infection which, in many cases, will attack only the feeder roots, but when larger roots are infected, only the inner bark is decayed and it does not discolor the wood. In addition, when observing the cross section of a dry root rot infected trunk, a grayishbrown discoloration in the wood tissue can be observed.
Dry root rot is a root disease, but symptoms of the root decline are seen above ground. They are similar to any of the root and crown disorders such as Phytophthora root rot, oak root rot fungus (Armillaria) and gophers. The trees lack vigor, leaves begin to turn yellow and eventually drop (especially in hot weather) causing twig dieback. Finally, the foliage will become so sparse that one will be able to see through the canopy of the tree. A period of two to three years may pass from the time of invasion until noticeable wilt. Many times, the tree will collapse in the summer, after a period of prolonged heat. In the case of dry root rot, the collapse is so rapid that the tree dies with all the leaves still on the tree. When looking for symptoms of dry root rot, keep an eye out for symptoms of other maladies as well — Phytophthora, oak root rot fungus and gophers being the most prevalent.
As mentioned previously, in order for Fusarium to infect a tree, there must be a predisposing factor such as girdling from gopher feeding. However, since many trees collapse from dry root rot without any apparent predisposing factor, there are obviously other factors which we have yet to identify. Therefore, in 1998, a grower survey was developed, along with intensive soil and leaf sampling, to attempt to identify as many new predisposing factors as possible. They might be elements in the soil, either deficiencies or excesses, or specific cultural practices such as irrigation patterns or fertilizer practices. Twenty orchards were identified from which 20 soil and 20 leaf samples were taken in diseased areas and another 20 soil and 20 leaf samples were taken from adjacent healthy areas. The owners or managers of the properties were given a questionnaire to complete regarding a variety of cultural operations. The objective was to identify those factors that would correlate well to trees becoming infected with dry root rot.
Survey Results
Soil analysis - The following laboratory procedures were conducted to see if there was any correlation between the disease and either deficiencies or toxicities of these elements or
conditions: sodium, boron, salt level, pH and soil type (sand, loam, clay). For these elements or conditions, no correlation was found. It would appear that for our sampling sites, these conditions, whether favorable or not (toxic or deficient), did not play a major role in predisposing the tree to dry root rot.
Leaf analysis - The following elements were analyzed for their concentration within the leaf: nitrogen, potassium, phosphate, manganese, magnesium and zinc. Of these, three correlations were found. Zinc and manganese levels were substantially higher in diseased trees. The third correlation showed a potassium deficiency in diseased trees. However, we do not believe that dry root rot is caused by elevated levels of zinc or manganese, or by potassium deficiency, but rather are a result of the disease. Unfortunately, it seems that we have still not identified any elements in leaf analysis that truly correlates and points to a predisposing factor for disease development.
Grower survey - The grower survey included questions on planting site (location, wind, previous crop, fumigation etc.), trees (source, type, rootstock, etc.), and cultural practices (irrigation, fertilization, gophers, history of Phytophthora, water quality, etc). Through statistical analysis it was found that the healthy and diseased sites were significantly different with reference to three conditions or situations: 1.) The presence of Phytophthora in an orchard will increase the chance of those trees succumbing to dry root rot. 2.) Orchards that have been fumigated have a less likely chance of succumbing to dry root rot. 3.) Balled vs. Container Plants -- growers were asked if their trees were balled or container
grown nursery plants. Healthy sites were significantly more likely to have been planted with balled trees (73% vs 33%). The results of this analysis were not strong, but rather they
suggest that there is a relationship between the disease and the type of tree planted - balled or container grown - and suggesting in favor of a balled tree for a healthy orchard.
Control Measures – What Works and What Does Not
Early experiments conducted by Menge, Ohr and Sakovich showed that the following circumstances or operations do not influence the incidence of this disease: fungicidal treatments, wounding the tap root at time of planting, sandy versus clay textured soils, spring versus fall planting and soil mounding.
- In choosing your nursery tree, the choice of rootstock is not important in that, as far as we know, all rootstocks are susceptible to this disease. However, since Phytophthora is a major component in dry root rot development, choosing a rootstock like sweet orange would certainly put those trees in a high risk category. We recommend that growers use Phytophthora resistant rootstocks like C35 or Citrumelo.
- According to the survey, it would be advantageous to fumigate before planting. Methyl bromide, although expensive, is the best fumigant as it is a complete biocide. If one chooses not to fumigate, the alternative would be a number of fungicide/nematicide applications to the newly planted trees. Generally speaking, this may work well with trees planted on a rootstock like Citrumelo or C35.
Phytophthora. Publications written in the 1970's, and again noted by our survey, showed that Phytophthora is a major culprit in the dry root rot complex. To control dry root rot, it is essential that the Phytophthora, when present, be controlled. This can be accomplished by fungicidal treatments, and by the proper application and timing of irrigation water. Overwatering creates a favorable environment for the multiplication of the Phytophthora fungus.
Gophers. It is well known that gopher damage provides entry points for Fusarium. Controlling gophers is an important factor in reducing the potential of infection by Fusarium.
Control
We presently have no direct control for dry root rot. To control the disease, we must control the predisposing factors such as gophers, Phytophthora, poor drainage and over-watering. If the predisposing factor(s) cannot be identified for a given diseased orchard, it will indeed be difficult to control the disease. Two things are certain though: 1.) There are no chemicals to date which will control this disease; and 2.) Presently, there are no rootstocks resistant to the disease.
Hear the latest on DRR with Akif Eskalen – a Webinar, July 24
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=30658
citrus dry root rot
Here Comes Citrus Dry Root Rot
Citrus Dry Root Rot
This impressive tree collapse is most noticeable after rainy season and the first heat waves after the rains
Citrus Dry Root Rot (July 24, 2019 from 3-4 pm)
Dr. Akif Eskalen, Plant Pathologist, UC Cooperative Extension Specialist, will discuss the symptoms, biology and management of citrus dry root rot. More information to come. One DPR CE unit (other) and one CCA CE unit (IPM) are pending.
Register in advance for the webinars by clicking on the event links above.
And if you missed it
Recording of the Management of Weeds in Citrus Orchards webinar is now available on YouTube - https://youtu.be/DU5bpRnq8DI
Dr. Travis Bean, assistant weed science specialist in UCCE, discussed the importance of weed management in citrus, tree age and variety considerations, scouting and weed identification, cultural and mechanical practices, and pre- and post-emergence herbicides.
Upcoming topics:
- Spray technology for tree crops (August)
- California Red Scale (September)
- Avocado diseases II. (October)
- Use of Plant Growth Regulators in Avocado (December)
Register in advance for the webinars by clicking on the event links above.
Are there Continuing Education units?
When the subject discusses pest or disease management, continuing education units will be requested from DPR (1 unit per session). Participants will pre-register, participate in the webinar and be awarded the unit. The sessions will be recorded and hosted on this web site for future study. However, continuing education units will be awarded only to the participants who attend the live version of the webinar.
Who is involved?
This webinar series is brought to you by Ben Faber (UC ANR Ventura Advisor) and Dr. Beth Grafton-Cardwell (Depart of Entomology UC Riverside Extension Specialist) with the technical support of Petr Kosina (UC IPM Contect Development Supervisor) and Cheryl Reynolds (UC IPM Interactive Learning Developer).
The Skinny on Avocado Pests and Diseases
Anthracnose, Sunblotch, and Armillaria Root Rot
Avocado Trunk Canker and Collar Rot
Avocado Branch Canker (Botryosphaeria)
Phytophthora Root Rot of Avocado and Management Strategies
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