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Nickels Lab-Related Work
On Lethal Phytophthora Canker, 1999 Greg
T. Browne Introduction This brief report on lethal
Phytophthora canker (LPC) work provides an overview of experiments originally
proposed for Nickels Estate but subsequently pursued in Kern County. The
location change resulted from concern of the principal investigator over
transporting the Phytophthora pathogens from Kern County to a
commercial growing area in Northern California. Although Phytophthora
cactorum and P. citricola are established in Northern California,
it is unknown whether populations of these pathogens in Kern County are in some
way unique or more virulent than the existing Northern California populations.
Incidence of tree mortality caused by LPC has exceeded 6% in several Kern County
orchards, but such losses are not currently documented in the northern almond
districts. Research objectives pursued with support of the Almond Board include:
1) determining the causal agents of LPC, 2) determining parts of almond trees on
which LPC infections occur, 3) determining sources of inoculum that contribute
to LPC, and 4) developing appropriate control measures for LPC. Experiments
summarized here relate only to portions of objectives 2) and 3). Complete
interim and annual reports are available from the Almond Board. Experiment on pre-infection control of LPC An experiment was established in
Kern County to determine the susceptibility of various tree parts to
LPC infection and whether Bordeaux 10-10-100 mixture or Nutri-Phite sprays can
protect against these infections. The chemical treatments were each applied to
five replicate four-tree plots on 29 October (tree age approx. 12 yr) . Three
weeks later the trees were inoculated on the trunk (by placing inoculum of P.
citricola under a bark patch or on the surface of uninjured bark); in the
scaffold crotch pocket (by placing the inoculum in the crotch, either with or
without chisel-inflicted injuries to mimic growth cracks); and on scaffolds (by
inoculating under a bark patch or on an uninjured latent bud). Control
inoculations were conducted in the same way, but with sterile "inoculant".
Assessment of treatment effects will be based on incidence and severity of
cankers caused by P. citricola; the first assessment will occur in
January 2000. Experiments on post-infection control of LPC Two experiments were conducted to evaluate post-infection efficacy chemical treatments for control of LPC. In the first test, P. citricola cankers were induced in a Kern County almond orchard by inoculating under bark patches on tree trunks and scaffold branches on 22 and 26 October. Controls received sterile inoculant. All sites inoculated with P. citricola developed cankers, but the controls remained healthy. On 1-3 December, canker size was measured, bark was prepared for post-infection treatments, and topical treatments were applied to the bark with paint brushes. The bark preparation treatments included 1) hatchet cuts that penetrated to the wood around canker margins, 2) surface bark removal around the canker margins, and 3) no treatment. The topical treatments included Aliette WDG® (1lb/gal), Ridomil Gold® (1 qt/3 gal), modified "magic elixir" (500 ml boiled linseed oil, 180 g basic CuSO4, 80 ml Hexol), and a non-treated control. Each treatment was applied to thoroughly wet the LPC-affected bark as well as adjacent tissue at least 10 cm beyond canker margins; healthy bark around the non-inoculated controls was treated at least 10 cm in all directions beyond the point of control inoculation. The first assessment of treatment efficacy will occur in January 2000. For a second experiment on post-infection control of LPC, branch segments were cut from healthy trees, inoculated with P. citricola on 22 November in a greenhouse, and incubated in plastic bags (non-inoculated controls were included). On 27 November, cankers were measured, bark was prepared for topical treatments, and the
chemical mixtures described above painted on the branch segments.
Bark-preparation treatments included 1) making chisel punctures into the wood at
least 10 cm beyond the canker margins or 2) no treatment, as a control. Final
canker dimensions were determined on 5 December. Each of the topical treatments
suppressed canker expansion, and the chisel cuts in the bark improved efficacy
of the elixer and Ridomil Gold (Fig. 1). After final canker measurements,
pathogen isolations from the margins revealed that P. citricola was
viable in all non-treated cankers and in most cankers treated with Aliette
without pre-treatment bark cuts or elixer with or without the bark cuts, but the
pathogen was usually not detected from cankers treated with Ridomil (Fig. 1). It
appears that chemical treatments may aid post-infection control of LPC cankers,
and additional work is planned to confirm the results and refine the treatments.
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