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Project description

Spatial Distribution and Water Relations of Armillaria mellea in Pear Orchards. (97FE048)
Program UC IPM competitive research grants program
D.M. Rizzo, Plant Pathology, UC Davis
K.A. Shackel, Pomology, UC Davis
Host/habitat Pears; Tree Crops
Pest Armillaria mellea
Discipline Plant Pathology
Applied Field Ecology
Start year (duration)  1997 (Three Years)
Objectives Evaluate the spatial distribution of Armillaria in commercial orchards and the relationship of the pathogen to tree crown symptoms and plant water status.

Determine the effects of irrigation type on the susceptibility of pears to Armillaria root disease in a controlled field study.

Determine the effect of soil moisture and temperature on growth of Armillaria under field and laboratory conditions.

A survey of plant water stress through an Armillaria infection center in Lake County showed clear patterns of stress associated with the borders of the infection center. These data indicate that, as expected, Armillaria infection causes water stress in the tree, and that this influence may extend well beyond the apparent limits of an infection center. Excavations of pear root systems confirmed the actual spatial distribution Armillaria on pear root systems and that rhizomorphs are the primary means of spread for Armillaria. The excavations have shown that Armillaria infection does not always start at the periphery of the root system; it often starts directly at the root collar or at scattered locations around the root system. To determine the effects of irrigation on disease development, a pear orchard was established at UC Davis in 1996. After two years, there was no clear difference in plant water status for trees grown under 3 different irrigation regimes, consisting of excessive sprinkler irrigation (high sprinkler), recommended sprinkler irrigation (low sprinkler) and recommended flood irrigation (flood). Rootstock type had a relatively small but significant influence on plant stress. The largest influence on water potential however, was a systematic difference between individual trees. To date, no affects of the Armillaria inoculations have been observed. The effect of moisture on the growth of rhizomorphs was tested in a field experiment. From first year results it appears that the drier upper soil surface in the flood irrigation treatment inhibits the growth of rhizomorphs.

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