, Department of Plant Pathology, University of Florida, Gainesville 32611; , School of Forest Resources and Conservation, University of Florida, Gainesville 32611; , Florida Forest Service, Forest Health Section, Gainesville 32608; and , School of Forest Resources and Conservation, University of Florida, Gainesville 32611
Silk bay (Persea humilis
Nash) is a member of the Lauraceae precinctive to the scrub forests of
central and southern Florida and a sister species to the primary laurel
wilt host, redbay (P. borbonia (L.) Spreng), which is generally
not found in these ecosystems. In November 2011, observations of silk
bay mortality near Lake Placid in Highlands County, FL, were reported to
Florida Forest Service agents. A subsequent visit to roadside and
homeowners' properties in the area revealed many dead and dying silkbays
with characteristic laurel wilt symptoms, including wilted crowns with
brown persistent foliage, frass accumulated at tree bases, sapwood with
dark streaking, and ambrosia beetle entrance holes (1). Twig samples
were taken and later confirmed as silk bay by the Florida Division of
Plant Industry in Gainesville. Trunk samples were taken from four trees
for fungal isolation. Stem sections with vascular discoloration were
surface disinfested for 30 s in a 5% sodium hypochlorite solution and
then plated onto cycloheximide streptomycin malt extract agar (CSMA)
(1). All sapwood pieces from the four samples resulted in the same
cream-buff submerged fungal growth characteristic of the laurel wilt
pathogen Raffaelea lauricola T. C. Harr., Fraedrich &
Aghayeva (2). DNA was extracted from a single-spore derived isolate,
PL1389, and an 18S small subunit ribosomal RNA gene sequence was
generated with primers NS1 and NS4, resulting in a 1,031-bp amplicon
(3). A BLASTn search showed identical homology to R. lauricola
strains PL159 and PL382 (GenBank Accessions No. EU257806 and JF797171,
respectively, 100% similarity, e-value 0.0, and a total score of 1,982).
The sequence was deposited into GenBank and assigned the accession No.
JQ247569. In December 2011, a spore suspension was made by flooding a
PL1389 culture plate with 2 ml of sterile water, collecting by pipette,
and quantification and adjusting to 3.25 × 106 spores/ml by
hemacytometer. Pathogenicity was tested on potted plants in a growth
chamber experiment. Five silk bays and three redbays were drill-wounded
with a 3/32” drill bit and inoculated with 20 μl of the spore
suspension. Three silk bays and two redbays served as water-inoculated
controls. Within 5 weeks, all inoculated plants displayed the wilt and
vascular discoloration characteristic of laurel wilt disease, while all
water-inoculated controls remained healthy. Sapwood samples from all
plants were plated onto the same CSMA media. R. lauricola was
later recovered only from the wilted plants inoculated with PL1389,
while no fungal growth was recovered from the asymptomatic
water-inoculated controls. Silk bay, which plays a significant role in
the limited scrub ecosystems of Florida, has now become another host in
the laurel wilt epidemic, with its implications upon the scrub forests
yet to be seen.
Read entire article at APS Journals.
