- Author: Ben Faber
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
https://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-07-17-1042-RE
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.
- Author: Ben Faber
The weather continues to offer new opportunities. Recently there have been reports of Avocado Brown Mite infesting trees. This is a pest (Oligonychus punicae) of dust and disturbance. Normally found along picking roads and up against areas that are dusty. Not a problem. But growers in San Diego and Ventura have both commented on their unusual presents and some cause for concern. It's early appearance in the season when the spider mite destroyer hasn't yet built its population is the most likely reason. But……………
Avocado brown mite is a sporadic pest, mostly in coastal growing areas. Bronzing of leaves, mite cast skins, and partial defoliation of some trees by avocado brown mite is most noticeable from about July to September. Severe infestations tend to occur in border row trees along dirt roads, where road dust is detrimental to mite predators. Ash deposited on leaves from wildfires reportedly also causes brown mite outbreaks.
Avocado brown mite feeds almost entirely on upper leaf surfaces. Bronzing damage is not severe when mite numbers are low to moderate (about 10 to 20 adult females per leaf). If the spider mite destroyer lady beetle (Stethorus picipes) is present and reproducing well at this time, brown mite does not normally become a problem. Damage occurs if avocado brown mite averages about 80 to 100 adult females per leaf (about 200-300 motile stages, adults and nymphs combined). At these higher densities mites also colonize the lower leaf surface and sometimes fruit, and partial defoliation can occur. These higher numbers cause leaf bronzing along the midrib, then along smaller veins, and finally the entire leaf turns brown.
Brown mite is related to Persea Mite (http://ipm.ucanr.edu/PMG/r8400211.html) but rarely causes the damage that its relative can cause. The Persea Mite feeds mainly on the bottom of the leaf and forms web-covered colonies which help to distinguish it from O. punicae (first described in Carthage?).
At this stage, unless there is severe defoliation, it might be best to leave it alone until biocontrol kicks in or we get a hot spell to burn it out. Keep the trees adequately watered and flushing new leaves to outgrow it. Call a PCA if you are worried.
More about the weather:
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26823
And whatever happened to the Bagrada Bug?
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=15198
Photo: Brown mite feeding on top of leaf surface
- Author: Ben Faber
It's that time of year to see some drama in avocado orchards. Once healthy-looking trees can suddenly turn brown in a weekend and all the surrounding trees still look fine. And it can be quite common in some years along the coast. The winter weather will have mild, cool even rainy days and then suddenly there's one of those 97 deg days and the tree goes down,
The entire tree or only one or several branches wilt suddenly when affected by Verticillium wilt. Leaves turn brown and die, but the dead leaves usually remain on the tree for several months. Brown to gray-brown streaks are visible in the xylem of the branches or roots when the bark is removed. Sometimes the streaking is visible in the branches, but often it is found at the base of the trunk.
Trees with Verticillium wilt often send out new, vigorous shoots within a few months after the initial wilting. If well cared for, affected trees often recover completely with no reoccurrence of the disease. However, not all trees survive an infection and disease symptoms sometimes reoccur after an apparent recovery.
The fungal pathogen Verticillium dahliae infects many hosts, including various berry and flower crops, cotton, eggplant, olive, pepper, stone fruit trees, strawberry, and tomato. Verticillium wilt is present throughout the state but is less common in avocado than root rot and canker diseases. Verticillium dahliae persists for years as microsclerotia in soil. Microsclerotia spread in infested organic matter and soil that is moved. The fungus infects through feeder roots, and then moves up in the water-conducting xylem system, restricting or preventing water movement to foliage from the roots.
No known methods are effective in curing infected trees. Trees often recover completely and display no further symptoms, even though they are still infected. After dieback ceases and new growth begins, prune off dead branches. Provide optimal irrigation and modest fertilization to promote new growth. If a tree dies from Verticillium, remove it. But give it a chance, there's a good chance it will recover.
In areas where V. dahliae is known to occur, plant Mexican rootstocks instead of the more Verticillium-susceptible Guatemalan rootstocks. Do not plant avocado on land where crops susceptible to Verticillium wilt have previously grown. Do not interplant avocado with other hosts of Verticillium, which are listed in publications such as Plants Resistant or Susceptible to Verticillium Wilt (PDF). Even if they have recovered, do not use trees infected with Verticillium wilt as a source of budwood or seed.
- Author: Ben Faber
Use of ProgGibb LV Plus® Plant Growth Regulator Increases Total Yield/ Fruit Size of Hass Avocados
As of March 27, 2018, foliar application of GA3 (ProGibb LV Plus®, Valent BioSciences, Corp.) to ‘Hass' avocado trees in commercial orchards has been approved. Dr. Carol J. Lovatt, emerita professor of plant physiology at the University of California – Riverside, recently completed research concerning the effectiveness of ProGibb LV Plus® on avocado fruit size and yield. A summary of her research — including best practices — follows.
Application Best Practices
ProGibb LV Plus® should be applied as a foliar spray when 50 percent of the trees in a block are at the cauliflower stage of inflorescence. If a grower cannot make an application at this time, it is best to apply the spray later, rather than earlier in order to ensure effectiveness.
The spray should be applied like a pesticide spray — full canopy coverage with a focus on the inflorescences. Those applying the spray should avoid spraying to run-off.
The ideal dilution for ground application is 12.5 fluid ounces of ProGibb LV Plus® (25 grams active ingredient [gai]) per 100 gallons of water/acre. For aerial application, use 12.5 fluid ounces (25 gai) in 75 gallons of water/acre. According to the research, the ideal application rate is 25g GA3 per acre; higher and lower doses were less effective. The pH of the water used should be adjusted such that the final pH of the spray solution is between pH 5.5 – 6.0.
Dr. Lovatt utilized organosilicone surfactant Silwett L-77® or Widespread Max® at a concentration of 0.05 percent as a wetting agent. Similar pure organosilicone type surfactants would be acceptable as wetting agents. It is important to note that until additional research can be conducted, other materials should not be included in the ProGibb LV Plus® spray solution.
Effect of ProGibb LV Plus® (GA3)
Dr. Lovatt's research team tested the effect of ProGibb LV Plus® on fruit size and yield for both ground and aerial applications. Overall, the research team noted that GA3 had no negative effects on ‘Hass' avocado fruit quality.
Ground applications were tested in March at groves located in Corona, Irvine and Somis, California. The tests were run on Duke 7 clonal rootstock trees at the cauliflower stage of inflorescence development. Each of the groves reported net increases in total yield and large/commercially valuable size fruit. The results were as follows:
Table 1. Effect of GA3 (25 g ai/acre) applied at the cauliflower stage of inflorescence development on yield and fruit size (pounds/tree) of ‘Hass' avocado trees in Corona, CA.
Treatment | Total Fruit | Net Increase (%) | Large fruit (213-354 g/fruit) | Net increase (%) |
---|---|---|---|---|
lb fruit/tree | ||||
GA3 | 74.7 az | 84 | 34.3 a | 128 |
Control | 40.6 b | 15.0 b | ||
P-value | 0.0997 | 0.0657 |
z Values in a vertical column followed by different letters are significantly different at specified P-values by Duncan's Multiple Range Test at the P-values indicated. (From the work of Salazar-García and Lovatt, 2000).
Table 2. Effect of GA3 (25 g ai/acre) applied at the cauliflower stage of inflorescence development on yield and fruit size as pounds and number of fruit per tree in an alternate bearing ‘Hass' avocado orchard in Irvine, CA.
Year 1 Yield | ||||
---|---|---|---|---|
Treatment | Total Fruit | Net Increase (%) | Valuable Size Fruit (178-325 g/fruit) | Net increase (%) |
GA3 | 92.2 az | 70 | 67.9 a | 65 |
Control | 54.2 b | 41.2 b | ||
P-value | 0.0029 | 0.0037 | ||
GA3 | 215 a | 76 | 141 a | 70 |
Control | 122 b | 83 b | ||
P-value | 0.0042 | 0.0026 |
z Values in a vertical column followed by different letters are significantly different at specified P-values by Duncan's Multiple Range Test at the P-values indicated. (From the work of Lovatt and Salazar-García, 2007; Zheng et al., 2011)
Table 3. Effect of GA3 (25 g ai/acre) applied at the cauliflower stage of inflorescence development at on yield and fruit size of ‘Hass' avocado trees in Somis, CA. Percent net increase reflects the benefit of GA3 at 25 g ai/acre relative to the untreated control trees.
Treatment | Total Fruit | Net Increase (%) | Valuable size fruit (178-325 g/fruit) | Net increase (%) | Large fruit (213-354 g/fruit) | Net increase (%) |
---|---|---|---|---|---|---|
lb fruit/tree | ||||||
GA3 | 408.1 az | 10 | 379.4 a | 13 | 294.1 a | 16 |
Control | 372.6 b | 335.3 b | 253.3 b | |||
P-value | 0.0626 | 0.0252 | 0.0626 |
z Values in a vertical column followed by different letters are significantly different by Fisher's Protected LSD test at the P-values indicated.
Overall, ground application of GA3 resulted in a net increase of 3,905 lb/acre, with a net increase of 4,851 lb/acre of commercially valuable size fruit (packing carton sizes 60+48+40; 178-325 g/fruit) and a net increase in large fruit (packing carton sizes 48+40+36; 213-354 g/fruit) of 4,488 lb/acre.
Aerial applications were tested on groves located in Pauma Valley and Carpinteria, California. Together, the aerial applications demonstrated that GA3 increased fruit set (fruit retention) by 55 percent into the last week of August and fruit size by 6 percent through mid-August.
Ultimately, Dr. Lovatt's research indicates that use of GA3 could result in substantial increases in net dollar return per acre to the grower due to increase in yield and commercially valuable size fruit. In addition, growers whose avocado groves are not suited to ground applications (groves located on slopes or in high-density formations) can benefit from the efficacy of utilizing aerial applications. In summary, ProGibb LV Plus® is “vital to the California avocado industry to increase grower income per acre to help sustain the California avocado industry.”
N.B. Remember, only well managed trees are going to respond. This will not turn around a poor producing orchard. only potentially increase production on an already good producing orchard. Ben
- Author: Ben Faber
Two questions and comments came up last week about the use of mulch in orchards. The first is that mulch is two edged. It serves to combat erosion and root rot, but it can also burn. Mulch and wood piled up against tree trunks and near trunks can cause damage to those trunks. A Fillmore grower actually goes through the orchard with a blower to move mulch away from trunks when alerted to fire. On the other hand, irrigated orchards have been shown to be effective at suppressing fire encroaching on homes.
So where to read more about fire? About mulch? Check out some of the blogs from the past.
Phire, Phlood, Phreeze and Fytophthora
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=22760
Mulch and green waste applied to avocado orchards and More
Green Waste, Yard Waste, Whatever You Call It- It has Simple Rules for Use
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=13093
A Safer Source of Inexpensive Orchard Mulch
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=12867
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=6458
Cellulase Production by Various Sources of Mulch
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=6809
A Caution on Free "Compost/Mulch"
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=11712
A Safer Source of Inexpensive Orchard Mulch
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=12867
Mulch, Avocados and the Home Garden
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=14906
Mature Compost Does NOT Kill Phytophthora
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19856
Use of Mulch in Organic Orchards Called into Question
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=21394