PHYTOPHTHORA CITRICOLA (SAWADA) – CAUSE OF AN IMPORTANT SHOOT ROT OF RHODODENDRON AND AZALEA

The first report on Phytophthora citricola Saw. (Mastigomycotina, Oomycetes) in Germany was given in 1959 after the fungus was isolated from diseased Rhododendron catawbiense. In and after 1979 the pathogen was recorded causing damage particulary to Rh. simsii, Rh. catawbiense, Erica sp. and Calluna sp. in nurseries.
Typical symptoms on Azalea are: palegreen discoloration of leaves, severe dropping of leaves on single shoots associated with brown discolorations inside the twig’s tissue.
Single twigs of Calluna turn brown from the tips downwards.
Usually no rot at the roots or shoot bases of diseased Ericaeae can be observed.

The disease cycle is still not clearly unterstood.
The fungus survives (Oospores) in the soil, enduring longer frost periods.

The infection takes place between 10 °C and 30 °C, primarily at the upper parts of the shoots.
This indicates that the spread of the disease may be due to zoospores, which are transported to the plant’s surface by drops of irrigation- or rainwater carrying contaminated soil particles.
In order to be successful, control programs in commercial plant productions have to combine sanitary steps and spraying programs using Fosethyl as a fungicide.

V International Rhododendron Conference

G.F. Backhaus

https://doi.org/10.17660/ActaHortic.1994.364.18