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.
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.
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.
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
Ambrosia beetles comprise a group of over 6,000 species in the Scolytinae subfamily. Most of these beetles typically attack decomposing and dead trees. The Polyphagous/Kuroshio Shot Borers have been reports on over 300 landscape and wildland living tree species, including avocado. Decline and death of trees has been noted in California since 2012, and the full economic extent is still unclear. The beetles feeds on a fungal symbiont that is introduced into the tree, and it is the fungus that spreads throughout the tree and causes the tree decline and death.
What was once thought to be another species of beetle (Tea Shot Hole Borer) and then identified as a new species - Polyphagous Shot Hole Borer- and now expanded to include another species of borer – Kuroshio Shot Hole – is showing that its fungal partners can be quite diverse. A recent publication indicates the increasing tangled association of the shot hole borer/disease complex that is affecting avocado and other tree species.
Two Novel Fungal Symbionts Fusarium kuroshium sp. nov. and Graphium kuroshium sp. nov. of Kuroshio Shot Hole Borer (Euwallacea sp. nr. fornicatus) Cause Fusarium Dieback on Woody Host Species in California
Francis Na, Joseph D. Carrillo, Joey S. Mayorquin, Cedric Ndinga-Muniania, and Jason E. Stajich, Department of Plant Pathology and Microbiology, University of California, Riverside, 92521; Richard Stouthamer, Department of Entomology, University of California, Riverside, 92521; Yin-Tse Huang, Department of Plant Pathology, National Chung Hsing University, Taichung 402, Taiwan, ROC, and School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville; Yu-Ting Lin and Chi-Yu Chen, Department of Plant Pathology, National Chung Hsing University, Taichung 402, Taiwan, ROC; and Akif Eskalen,† Department of Plant Pathology and Microbiology, University of California, Riverside, 92521
Shot hole borer (SHB)-Fusarium dieback (FD) is a new pest-disease complex affecting numerous tree species in California and is vectored by two distinct, but related ambrosia beetles (Euwallacea sp. nr. fornicatus) called polyphagous shot hole borer (PSHB) and Kuroshio shot hole borer (KSHB). These pest-disease complexes cause branch dieback and tree mortality on numerous wildland and landscape tree species, as well as agricultural tree species, primarily avocado. The recent discovery of KSHB in California initiated an investigation of fungal symbionts associated with the KSHB vector. Ten isolates of Fusarium sp. and Graphium sp., respectively, were recovered from the mycangia of adult KSHB females captured in three different locations within San Diego County and compared with the known symbiotic fungi of PSHB. Multigene phylogenetic analyses of the internal transcribed spacer region (ITS), translation elongation factor-1 alpha (TEF1-α), and RNA polymerase II subunit (RPB1, RPB2) regions as well as morphological comparisons revealed that two novel fungal associates Fusarium kuroshium sp. nov. and Graphium kuroshium sp. nov. obtained from KSHB were related to, but distinct from the fungal symbionts F. euwallaceae and G. euwallaceae associated with PSHB in California. Pathogenicity tests on healthy, young avocado plants revealed F. kuroshium and G. kuroshium to be pathogenic. Lesion lengths from inoculation of F. kuroshium were found to be significantly shorter compared with those caused by F. euwallaceae, while no difference in symptom severity was detected between Graphium spp. associated with KSHB and PSHB. These findings highlight the pest disease complexes of KSHB-FD and PSHB-FD as distinct, but collective threats adversely impacting woody hosts throughout California.
Even though it's been a mild winter; other than fire, rain and some cool nights, we did have a few days of hot weather, which is just what sets off citrus dry root rot. It pops up after the first hot weather of the spring, and there it was driving down the road. There's nothing that can be done with this tree. Tree removal and replacement is the answer. Prevention is the solution. We know that Fusarium fungus is usually associated with the collapse, but wounding is the key. Mechanical injury from weed whips, discs, gophers, voles, rabbits. Wounding from salt damage might do it too.
Dry root rot, a disease caused by the soil fungus Fusarium solani, has a long history of hindering production of citrus in California. Fusarium solani is a weak pathogen that infects only when there is some kind of stress in citrus tree. However, presence in almost all citrus orchards everywhere in the world including California, enhance its quick exploitation of such stresses. Some of the factors that are possible stress include invasion by other pathogens, such as Phytophthora and Citrus Tristeza Virus (CTV). Other stresses are wounding by gophers/rodents or insects, girdling, asphyxiation especially drowning the tree with too much water, soil nutrient content and fertilization, irrigation, and other cultural practices in the orchard. The pathogen is an opportunist on citrus. With the stress, Fusarium solani begins infection by colonizing the cortical tissue of feeder roots, advances into the lower tap root and/or scaffold root, and move up through the bud union into the center of the trunk. Studies over the years have shown that many rootstocks are susceptible and old trees as well as young twigs are not spared of the disease.
Photo: collapsed tree near and older one on the right, but also near a younger one that probably was a replacement for one that had collapsed several years prior
At a recent conference in Florida, University of Florida entomologist Daniel Carrillo reported some very disturbing news. There is a fungus/pest complex in Florida avocado and related native laurel species that is similar to a complex found in the California - Shot Hole Borer/Fusarium fungus complex. There it is called Laurel Wilt Disease and is a complex of the ambrosia beetle Xyleborus and the fungus Rafaelea lauricola. It is a fungus/insect complex that causes death in avocado and the relatives of avocado trees. The California complex can cause the death of many tree species, such as sycamore, coast live oak and willow, as well as the decline of avocado. The complex and disease are called Fusarium Dieback here, caused by Euwallacea ambrosia beetle and Fusarium fungus.
What Carrillo and other colleagues have found is that there are similar species to their introduced Rafaelea species of ambrosia beetle that are now attacking live avocado trees. These so-called cryptic species are members of a group of beetles that normally do not attack live trees. These beetles are typically some of the first group of decomposers that go after dead trees. These newly identified insects are morphologically very similar to the original beetle, but are native members of the Florida environment. They too are now attacking live trees. There are now ten potential species of ambrosia beetle that can introduce pathogenic fungi.
To exacerbate the situation, there are other fungi now that have been associated with these beetles that may be similarly as pathogenic as the original fungus. These fungi are genetically distinct from the species causing damage in California. However, this ability of different fungi to adapt to a new invasive beetle species and the ability of other beetle species to pick up the pathogenic fungal species is a scenario that might appear in California.
As the world becomes smaller and more living materials are moved around and they mix, this may be the new reality we are facing.
To read more about the Florida findings check out the article:
There's a lot of work being done in Florida on a pest/disease complex like we have here with Shot Hole Borer and Fusarium fungus – Fusarium Dieback. This hits avocados and a lot of other native trees like sycamore, willow and coast live oak. Some of the success in Florida may be applicable and we are working with their researchers there to adapt some of the techniques here.
Redbay ambrosia beetle, Xyleborus glabratus, is a wood-boring pest that has now invaded nine states in the southeastern United States. The beetle's dominant fungal symbiont (Raffaelea lauricola) is phytopathogenic, inducing laurel wilt in trees within the family Lauraceae. Members of the genus Persea are particularly susceptible to the lethal disease, including native redbay (P. borbonia) and swampbay (P. palustris), as well as commercial avocado. Cubeb oil lures are the current standard for detection of X. glabratus, but recently eucalyptol and a 50% α-copaene oil have been identified as additional attractants. This study used a combination of choice bioassays, field cage release-and-recapture assays, and a 12-week field trial to compare efficacy of eucalyptol and copaene lures relative to commercial cubeb lures. In addition, gas chromatography (analyzer for volatiles) was used to quantify emissions from lures field-aged for 12 wk. In field cage assays, copaene lures recaptured a higher percentage of released beetles than cubeb lures. In the field test, cubeb lures captured fewer beetles than copaene lures, and lowest captures were obtained with eucalyptol lures. Both copaene and cubeb lures were effective in attracting X. glabratus for 12 weeks, but field life of eucalyptol lures was only 4 weeks, consistent with the quantification of lure emissions. Results suggest that the 50% α-copaene lure provides the best pest detection currently available for X. glabratus.
So why is this important? For one, it will serve as a tool for more effectively monitoring the presents of the beetle. But more importantly, it might be used as a lure to attract them away from trees. Fool them into going somewhere else, like to die.