- Author: Mary Lu Arpaia
- Author: David Obenland
Over the last two weeks Mary Lu Arpaia (Extension Specialist, UC Riverside) and David Obenland (Plant Physiologist, USDA-ARS) had the opportunity to visit Israel and spend several days looking at avocados including a visit with Zvi Mendel and Stanley Freeman, the lead researchers in Israel on the Polyphagous Shot Hole Borer (PSHB) and its Fusarium fungal symbiont. This is their report.
So far the beetle is still largely confined to the central coastal region of Israel and the northern Negev. The beetle has also been found in the Upper Galilee at Kibbutz Hagoshrim in avocado and on ornamental trees in other locations in this region which is quite far from the primary infested area. The infested avocado trees have been destroyed but the beetle population already spread outside of the site of the initial infestation. Interestingly, this find was with a grower who packs their avocados in the coastal area. It is assumed that the beetles arrived in the bins originating from the infested area. This is a reminder to California growers and packers that to minimize the spread of pests clean bins are essential. The spread of avocado thrips and persea mite in California is also assumed to have been by bins containing vegetative material.
The Israeli researchers have continued searching for materials that will either control the beetle or the fungus. They have had reasonable success in the lab when they test materials under controlled conditions but application out in the field is not effective. There are no chemical treatments on the horizon that growers can use.
We visited infested avocado orchards in the Hefer Valley and the region southwest of the Carmel mountains (south of Haifa). We visited a Reed orchard which is believed to have been infested approximately 5 to 6 years. Three years ago this grove showed heavy infestation in the entire grove. What we saw on our visit was severe limb dieback, many broken branches scattered on the orchard floor, dropped mature fruit and smaller than normal fruit size for the fruit remaining on the trees. Signs of the beetle boring as evidenced by sugar exudates were easy to find wherever we looked. Dr. Mendel told us that the grower is giving up on this orchard and plans to bulldoze the orchard after harvest. We went on to see several other groves; in all except a 2-year-old orchard it was easy to find limb dieback, fallen fruit and sugar exudate up and down branches. We were told that they do not often find infestation of young groves but when they do, it is usually on the base of the trunk (either rootstock or scion).
We visited a plot with some growers along with Leo Winer (an extension officer) and Udi Gafni (head of the research and development unit of GRANOT) where insecticide applications to infested trees had been made last fall (2012). Unfortunately, signs of continued beetle activity were relatively easy to find. The growers told us about seeing fruit shriveling as the branch dies back. Since substantial fruit drop occurs of both mature and developing there is also an overall drop in productivity as an infestation spreads throughout a grove. Growers are extremely concerned and frustrated that there are no control measures for the beetle. Similar to California, Israel has historically used minimal pesticide sprays. The growers know that in areas already infested that spray applications are key to their continued orchard viability.
Avocado growers in Israel are also seeing problems with Botryosphaeria fungal infections. We visited the northern Negev Desert area where avocados are grown. We went to a large Hass orchard the grower is attempting to control this problem using phosphite injections. We were able to see upper limb dieback and staining on the upper branches.
In the meantime, Drs. Mendel and Freeman are continuing to study beetle biology and the behavior of the Fusarium fungal component. Dr. Mendel is developing a method to raise the beetle in the laboratory. This will be a important breakthrough since it will allow for a better understanding of the beetle life cycle. They know that the female beetle once it flies and seeks a place to burrow has about a 48 hour window to successfully establish itself in the host plant since this is the time period it can survive without feeding. We also learned that the beetle carries the Fusarium spores in its mycangium (a specialized structure at the back of its jaw), rather than hyphae. The larvae and pupa do not have mycangium, only the adults. While the larvae pupate, the Fusarium in the galleries sporulates and the emerging adult as it feeds picks up the Fusarium spores. On a side note, Drs. Mendel and Freeman do not necessarily agree with our use of the name PHSB. They argue that the beetle is monophagous (eats only one kind of food, Fusarium) but uses several tree species as hosts. We will have to see how the final name for the beetle is ultimately settled among the insect taxonomists.
Finally, Dr. Mendel is very worried about the spread of the beetle to native tree species in Israel, especially oaks which are found in many areas throughout the country. The box elder (which also occurs as a landscape tree in California) has been decimated by the PHSB. This is a warning flag for all California residents to take action to safeguard our native oaks, other native species and landscape trees.
These visits reinforced the extreme importance that the California industry must be diligent looking for tree infestation. The industry must work with the landscape industry and forestry service in southern California to understand how fast the infestation is spreading and what hosts are most susceptible. The industry needs to continue funding both applied (surveys, control measures and understanding of the beetle and fungal biology) as well as more basic work such as the origin of the beetle. This latter piece of information may lead to better ideas concerning control measures. Finally California researchers and industry leaders would be well advised to collaborate closely with our Israeli colleagues. They are facing this crisis head on since the PSHB has already spread into commercial groves. From their advanced experience, the California industry can learn much.
A conference on the PSHB and its Fusarium fungal symbiont was held in August 2012 in Riverside. The talks from this meeting are available for review on the website www.avocadosource.com,
. Video of the conference presentations from the public meeting are available for viewing on YouTube. The science portion of the meeting will be posted on YouTube in the forthcoming weeks.
Figures 1,2. ‘Reed’ avocado tree in the Hefer Valley, Israel. Note previous limb breakage and branch dieback due to infestation by the PSHB and its fungal symbiont, Fusarium. (Pictures taken 3/9/2013 by M.L. Arpaia).
Figure 3. Infested ‘Reed’ branch in later stages of decline. The white larvae are termites. Once the branch begins to dieback, termite infestation usually follows. Note the staining of the wood and evidence of the PHSB galleries. (Picture taken 3/9/2013 by M.L. Arpaia)
Figure 4. External symptoms of infestation including sugar exudate and bark darkening reveals wood staining due to Fusarium infection of the woody tissue. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 5. Small branch infestation and dieback in ‘Hass’. Picture taken at Kibbutz Ma’agan Michael along the central coast of Israel. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 6. ‘Hass’ avocado tree in the northern Negev affected with Botryosphaeria. Note the dieback and the fact that previously infested wood has been removed. (Picture taken 3/9/2013 by M. L. Arpaia)/h3>
- Author: Ben Faber
Invasive Ambrosia Beetle/Fusarium Complex
You are invited to attend a public meeting about the invasive ambrosia beetle/Fusarium complex that are threatening avocado, oak, sycamore, persimmon, and box elder trees in California.
August 14, 2012
2:00 - 5:00 pm
Preregistration is required but there is no fee to attend. There is, however, a $4.00 parking fee.
Below, picture of the cankers caused by the ambrosia beetle and where the insect starte growing the fungus which will eventually kill the tree.
- Author: Neil O’Connell University of California Cooperative Extension, Tulare County
Dry root rot has been a problem in citrus orchards for many years. Although generally a problem in coastal and northern California counties it has been reported in other citrus producing areas of the state. When present it generally occurs as a chronic problem affecting only a few trees in the orchard. Trees may be invaded at any time from planting to maturity; frequently mature, good producing trees are invaded. Once infection has occurred, it may be several years before any symptoms are visible in above-ground portions of the tree. Symptoms may be a gradual leaf drop and twig dieback or a sudden death of leaves which dry and remain in place. The tree rapidly collapses as a critical mass of roots is damaged or the crown area is girdled.
Investigations of declining trees in the past revealed decaying bark in the root system and/or crown area of the tree which was thought to involve brown rot gummosis caused by Phytophthora invasion. The decaying bark area eventually dried and cracked. No gumming was observed, however, as is typical of brown rot gummosis. A grey staining of the woody portion of root or crown tissue was observed which is not seen with Phytophthora where only the cambium tissue is affected. Further investigations by researchers revealed that in affected tissue in these declining trees Fusarium solani could be isolated. Other organisms including bacteria and weak parasites and saprophytes could be isolated as well.
Tissue samples from affected trees have consistently yielded Fusarium spp. Microscopic examination of affected areas revealed a plugging of the water conducting xylem tissue. During high temperatures, this plugging could result in slight wilt or rapid collapse of the tree depending upon the percentage of water conducting elements affected in the roots or crown area.
Early investigations in declining orchards identified stress factors which seem to predispose the tree to invasion by the organism which is not possible without one or more of these agents. Stress factors identified included environmental factors such as drought, cultural such as damage from fertilizer, herbicide, nematicide or waterlogging, and damage from rodents such as gophers. Chemical agents applied at critical periods or in excessive amounts appeared to be stressful to affected tissue thus rendering it susceptible to invasion. Water ponding next to the trunk of the tree or waterlogging of the roots was associated with invasion of root or crown tissue and later colonization by this wood rotting organism. Stress produced in the tree together with the presence of the dry root rot organism is thought to predispose the tree to invasion of the organism.
Research involving the mechanisms of invasion of Fusarium involved exposure of seedlings to hot water and then the dry root rot organism which resulted in invasion where exposure to the organism without previous exposure to high temperature did not result in invasion. It was hypothesized that high temperatures may have interfered with natural defense mechanisms allowing invasion. Research has identified a relationship between Phytophthora and the vascular wilt causing Fusarium spp. Phytophthora lesions on roots favored the invasion of the Fusarium. Seedlings exposed to only the wilt causing organism were not invaded, but were invaded if exposed to Phytophthora and then the wood rotting organism. A relationship was established between temperature and invasion of Phytophthora. Seedlings were not invaded by Phytophthora in a medium at 75 or 65 degrees but were at 55 degrees. Results suggested that the seedling formed scar/callus tissue capable of excluding the organism at higher temperature but was unable to do so at the lower temperature.
While most commercial rootstocks possess a moderate to high degree of tolerance to Phytophthora invasion, all rootstocks are thought to be susceptible to the dry root rot organism.