Laurel Wilt Disease Overview — Impact in Florida and Threat to California

  • Aug 14, 2019

In 2002, packing material infested with the redbay ambrosia beetle led to the introduction and spread of Laurel Wilt Disease (LWD) from Georgia, to Alabama, Arkansas, Florida, Louisiana, Mississippi, North Carolina, South Carolina and Texas. The disease, which is caused by the Raffaelea lauricola (RL) fungus, is transmitted by the redbay ambrosia beetle — a pest that is rapidly spreading across the southern U.S.  The pest is easily spread by the transfer of infected wood to other areas. Thus far, no economically viable chemical control measures have been identified.

Recently, a group of Florida researchers shared their LWD research with California avocado growers in a series of seminars in Fallbrook, Ventura and San Luis Obispo. Following are highlights from their presentations. Links to all of the presentations can be found at the end of this article.

Impact on Florida Avocado Industry

  • LWD has been detected across all of Florida and in every avocado production area
  • Of the 125 square miles of the avocado industry, about 1,200 acres (120,000 commercial avocado trees) were lost to LWD for an overall economic impact of $100 million
  • Estimated $42 million in tree loss value and approximately $4 million loss in avocado sales

LWD Epidemiology

  • LWD is very virulent and it only takes one beetle to inoculate and kill a tree because the average beetle carries more than 6,000 spores
  • Spores are carried within the beetle’s mycangia (“pockets”)
  • RL fungus invades the tree’s xylem, making the xylem nonfunctional and thus foliage dies due to a lack of water
  • All avocado cultivars are susceptible to RL, but the rate of decline varies across cultivars based on the size of xylem vessels (the higher the sap flow rate, the greater susceptibility to LWD)
  • West Indian cultivars are the most susceptible
  • Non-grafted, clonal ‘Toro Canyon’, ‘Dusa’ and ‘Duke-7’ are susceptible, but disease progression is relatively slow
  • LWD progresses faster in larger trees
  • Disease also spreads to other trees via root grafts and results in multiple dead trees within a row
  • Several species of ambrosia beetles now carry R. lauricola and can transmit it to avocado trees

LWD Symptoms

  • Earliest symptom is green wilted leaves in portions of the canopy
  • Leaves quickly become brown and remain attached to the stem
  • Stem and leaves die back until the tree eventually dies (death is within weeks)
  • Trunks/limbs may have small holes with sawdust tubes
  • Unhealthy sapwood color with blue/black streaks
  • Boring galleries

LWD Monitoring and Management

  • If no management steps are taken, 100 trees can be lost within six months
  • Ambrosia beetles attack stressed trees, therefore it’s important to carefully monitor the health of trees
  • Ambrosia beetles prefer dense plantings, overcrowded and overlapping branches, thus make the crop environment less suitable to the beetles by topworking and pruning trees and removing dead branches with the purpose of increasing the amount and hours of sunlight
  • Look for early symptoms of LWD (green leaf wilt in portions of the canopy) and immediately remove any infected/infested trees
  • All infected tree materials (limbs, stump, roots) must be removed and destroyed (chip, shred or burn)
  • To protect trees around a removed, infected tree apply tree-directed spray of a registered pesticide and adjuvant on trees in a one-acre area adjacent to the affected tree
  • Trees adjacent to an infected tree also can be hatracked or stumped — AB are less attracted to trees with small wood diameters and high light regimes

The following presentations are available online.