Posts Tagged: plant pathology
In 2003, the Polyphagous Shot Hole Borer (PSHB) was observed in Southern California. This beetle has a Fusarium sp. symbiont that causes susceptible host tree damage. The PSHB appears to be established in Los Angeles, Orange, and Riverside Counties. In the Los Angeles area, PSHB has attacked over 200 species of native, ornamental and horticultural trees, including the native Coast Live Oak, California sycamore, and about 57% of the commonly used street trees in the area. Avocado trees are susceptible; the beetle and fungus have been found in several backyard avocado trees as well as some commercial avocado orchards in Los Angeles and Orange counties. The PSHB has caused severe damage to commercial avocado orchards in Israel, and is a threat to California’s avocado crop which is worth about $460.6 million and accounts for about 87% of the US production.
To better understand the biology of and potential management strategies for the PSHB Fusarium complex threatening California’s avocado trees, Mary Lu Arpaia, Subtropical Horticulture Extension Specialist in the UC Riverside Botany and Plant Sciences Department and Kearney Agricultural Research and Extension Center, and David Obenland, Plant Physiologist, at USDA-ARS in Parlier, California, visited Israel in March, 2013. The PSHB is an invasive pest in Israel and was found in Israel’s commercial avocado orchards in 2009. The PSHB in Israel is causing serious damage, and Israeli research may help us develop detection and management strategies in California.
Biology and Behavior: A visit with Drs. Zvi Mendel and Stanley Freeman, Israel’s lead PSHB researchers provided insight into research on the biology and behavior of the PSHB Fusarium complex. Drs. Mendel and Freeman contend that even though the beetle uses several tree species as hosts, it is monophagous, only feeding on the Fusarium symbiont. According to their research, the beetle’s reproductive cycle in susceptible trees is: the adult pregnant female beetle bores through a tree’s bark, creates galleries under the bark, infests the galleries with a Fusarium fungal symbiont carried in the adult beetle’s mycangium (a specialized structure at the back of the jaw), and lays eggs in the galleries. Larvae are mostly female. While the larvae develop, the Fusarium sporulates. The developing larvae eat the fungus, develop into adults in about a month, and mate while in the gallery. The pregnant females exit the gallery through exit holes in the bark. To survive, the emerged pregnant female has about 48 hours to find a suitable host location and continue the life cycle.
Hygiene: The geographic area where the beetle is found in Israel appears to be expanding, even though the infested avocado trees in the primary infested area were destroyed. The PSHB is thought to have been transported in bins originating from the infested area. A best practice for California growers and packers will be to use clean bins and ensure that there is no vegetative matter in the bins prior to transport. This practice will also help reduce the spread of avocado thrips and persea mite.
Chemical control of the beetle or the fungus: There has been limited success in controlled lab situations, but field applications are not effective. So far, no effective chemical control technologies have been developed for the beetle.
Infestation: A mature orchard that was infested approximately 5 to 6 years ago became heavily infested within 2 to 3 years, and will be bulldozed. Infestations may require bulldozing orchards.
Detection: Mature avocado orchards with heavy infestation have severe limb dieback, many broken branches littering the orchard floor, dropped mature fruit, abnormally small mature fruit, and sugar exudates caused by the bore holes. A 2-year-old infested orchard did not exhibit very much limb dieback; when infestation of young trees is observed, it is usually on the base of the trunk (found in either the rootstock or the scion wood).
Spread to native and landscape hosts: Israel has seen the spread of the beetle to native oaks and the box elder. California will need to educate the public to help safeguard our native and landscape trees from the spread of the PSHB and Fusarium complex. Native and landscape tree infestations may make it difficult to control the spread of infestations into commercial avocado orchards.
California outlook: California must be diligent in looking for tree infestation. The avocado industry is working with the landscape industry and forestry service in California to understand the rate of infestation and range of susceptible hosts. Funding for surveys; understanding the beetle and fungal biology; developing control strategies, technologies and practices; and basic work on the origin of the beetle is essential. California researchers should collaborate with and learn from their Israeli colleagues.
Avocado tree in Israel with limb breakage and branch dieback due to PSHB infestation and its Fusarium symbiont (photo by M. L. Arpaia).
External symptoms of PSHB infestation, including sugar exudate, bark darkening, and wood staining due to Fusarium infection of the woody tissue (photo by M. L. Arpaia).
PHSB infested avocado branch in later stages of decline. The white larvae are termites that infested the branch after dieback started. Note the staining of the wood and evidence of the PHSB galleries. (photo by M.L. Arpaia)
KARE scientist visits Australia to share insights into disease control and food safety strategies for tree nut crops.
Themis Michailides, plant pathologist and lecturer in the Department of Plant Pathology at UC Davis, and Kearney Agricultural Research and Extension Center, recently visited Australia, primarily to visit pistachio and almond orchards and discuss disease control and food safety strategies for these crops.
In 2011, Australia had excessive rains at harvest time, which resulted in pistachio crop losses of 40 to 50 percent due to anthracnose fungi. The lost crop was worth about $15 million. To help prevent the problem in the future, the growers went to California to get input on current disease management strategies. Themis Michailides’ research and extension program was very helpful. As a result, the pistachio and almond crop growers of Australia created protocols to prevent and control the disease. The Australian Pistachio Industry invited Dr. Michailides to tour the Riverland and Sunraysia regions this year to inspect orchards that were previously affected by the anthracnose as well as meet with Australian researchers.
Dr. Michailides was surprised to find lower limb dieback in Australia. This is a problem that his program studied in California for many years with funds from the Californian Almond Board.
The trip was mutually beneficial. The Australian industry and researchers received expert advice and Dr. Michailides learned about practices in Australia that can benefit his research and extension program in California.
Highlights of the trip are summarized below.
- Brisbane: met with Australian plant pathologists from the local area and discussed many plant diseases of interest to the local region.
- Adelaide: stayed and enjoyed visiting with his UC Davis classmate Prue McMichael’s family; visited local laboratories and research organizations to discuss pistachio, pomegranate and almond disease control and food safety strategies.
- Mallee and Riverland regions: met with growers and researchers to discuss disease control and food safety strategies for pistachios and almonds. Displayed samples of Anthracnose and Botryosphaeria infected nuts and leaves that were collected during Dr. Michailides’ Australian orchard visits.
- Mildura: met with the Australian Pistachio Research and Development Committee and discussed some of the pistachio disease control efficacy research being done in Australia. Recommended that the committee access “Fungicides, Bactericides, and Biologicals for deciduous Tree Fruit, Nut, Strawberry, and Vine Corps 2012” to review current pest management strategies for tree fruit, tree nut, strawberry and vine crops. Shared how to closely inspect trees, bark, wood, leaves and nuts to identify and diagnose symptoms of various diseases, as well as predict disease risks based on inoculum levels and weather conditions.
Dr. Themis Michailides inspecting pistachio trees in Australia for disease symptoms. (Photo courtesy of The Murray Pioneer Pty. Ltd.)