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
The USDA has released their Fruit and Nuts Outlook Report which shows the forecast the 2019/2020 seasons and provides an overview of the markets.
The 2019/20 citrus crop is forecast to be 7.63 million tons, down 4 percent from the previous season. Declines in overall production can mostly be attributed to smaller lemon, tangerine, and mandarin crops in California. Orange production in California has remained stable since last season. Citrus production in Florida has also remained stable with a 1 percent decline in orange production, and significant increases in grapefruit, tangerine, mandarin, and tangelo production over last year. Overall decreases in production of lemons, tangerines, mandarins, and tangelos are expected to result in increased imports, and higher prices compared with last year.
Fruit and tree nut grower prices began 2020 at low levels. At 117.8 (2011=100), the January 2020 index was down 10 percent from the January 2019 index and below the January average for 2016-18 (fig.1). The January 2020 index was the lowest since January 2013. Significantly lower grower prices for citrus fruit and apples drove down the index (table 1).
As of mid-March 2020, U.S. citrus exports were down except for orange juice and tangerines. Reduced exports have increased the domestic supply of citrus, putting downward pressure on prices. The January 2020 price of all- grapefruit is down 36 percent from the year before, and all-oranges and oranges for the fresh market are down by 6.9 and 9.4 percent respectively. All- lemon prices are down 28.5 percent, and fresh lemons prices are down by 8.6 percent.
Apple prices were down 21 percent in January 2020 from the year before. USDA, National Agricultural Statistics Service (NASS) estimates the 2019 total apple crop to be up 3.6 percent from 2018. The strong dollar and increased tariffs in several countries have reduced exports, putting downward pressure on prices.
On Thursday, October 17, 2019 the University of California Cooperative Extension (UCCE) held a Lemon Rootstock and Scion workshop and field tour in Santa Paula, California. Results from Citrus Research Board (CRB) sponsored field trials evaluating several lemon rootstocks and scions were highlighted at this meeting. This event was well attended by over 100 grove managers, citrus growers and industry representatives. Speakers included CRB-funded researchers Ashraf El-kereamy, Ph.D., Mikeal Roose, Ph.D., Tracy Kahn, Ph.D., and Glenn Wright, Ph.D. The CRB has been funding research on several single site and multi-location lemon field trials, some of which have been ongoing since the late 1980's. Mikeal Roose, Tracy Kahn and Glenn Wright provided updates on the results of previous rootstock trials and the multi-location lemon rootstock and scion trials at Santa Paula, Lindcove Research and Extension Center and desert locations. Attendees learned about the various yield, packout and fruit quality of several lemon selections being studied in addition to new, non-lemon citrus cultivars to be released in the coming years. Following the presentations, attendees were invited to visit one of the lemon rootstock field trials.
Limoneira Company has been extremely helpful in much of this lemon research over the years. We thank them and also the Master Gardeners who helped in organizing the event held at the Limoneira meeting center. Read about a sideline at the event HERE.
Presentations from the workshop are available on the UCCE Ventura website HERE.
Lemon Scions and Lemon Rootstocks Workshop & Field Tour
October 17, 2019
Visitors Center, Limoneira Co., 1141 Cummings Rd, Santa Paula, CA 93060 View map/directions here
Contact: Ben Faber, 805-645-1462, firstname.lastname@example.org
Which Way Lemon Industry?
What are Lemon Rootstock Choices?
What are Lemon Scion Choices?
How are they Complementary?
What are their Effects on Yield? Fruit Quality? Canopy Growth? Soil Compatibility? Tree Health?
HLB Tolerance? Maybe We Know a Little.
How do Scions and Rootstocks Compare in the Desert, Valley and Coast?
Come Learn this and more. Tour a Scion/Rootstock Trial
Mike Roose, UC Riverside Citrus Breeder
Tracy Kahn, UC Riverside Curator Citrus Collection
Glenn Wright, University of Arizona Citrus Extension Specialist
Ashraf El-Kereamy, UC Riverside Citrus Specialist
According to the latest USDA Foreign Agricultural Service GAIN Report (Global Agricultural Information Network), the European Union is still a major citrus producing area. EU citrus production is concentrated in the Mediterranean region. Spain and Italy represent the leading EU citrus producers, followed by Greece, Portugal, and Cyprus. For MY (October/September) 2018/19, Post expects overall citrus production to grow mainly in Spain due to favorable weather conditions. The quality of the fruit is forecast to be excellent and EU domestic consumption of citrus may stay flat in 2018/19.
EU lemon production is forecast to grow 10 percent and is stable compared with previous estimates. The overall growth is due to the strong production rise expected in Spain, the largest lemon EU producer. According to the latest data from the Spanish Ministry of Agriculture, Fisheries, and Food (MAPA), Spain's 2018/19 lemon production is forecast at 1.1 million MT, an increase of 19 percent compared to the previous year. Favorable weather conditions resulted in good flowering and fruit setting. In addition, in recent years Spain has increased its total planted area for lemons. Fruit quality is forecast to be excellent. ‘Fino' lemon is expected to increase by 14 percent due to the entry of new plantations over the last years. ‘Verna' lemon is expected to rebound; increasing by 90 percent as production of ‘Verna' lemon in the previous season was shorter than normal levels. Spain will continue to consolidate its leading commercial position in Europe with quality and phytosanitary guarantees. Following Argentina, Spain is the second largest lemon producer in the world but the first global exporter of lemons for fresh consumption. Spanish lemon production is concentrated in the regions of Murcia and Valencia, and the Provinces of Malaga and Almeria in Andalusia. ‘Fino' and ‘Verna' are the leading lemon varieties grown in Spain, accounting for 70 and 30 percent of the total production, respectively. The ‘Fino' variety is predominantly used for processing.
So far, Asian Citrus Psyllid and HLB are not a problem in the lemon producing areas of Spain and Italy. Read more about the citrus industry in the European Union – oranges, grapefruit, mandarins, fresh, processed, policy, export issues, MRLs and tariffs. Fascinating stuff and the potential impacts it has on California growers and production.
And what about what's going on in the Moroccan citrus world, right next door to Spain?