Diseases of citrus (
Phytophthora foot rot or gummosis
Phytophthora foot rot, also known as Phytophthora gummosis, is caused by two soil borne organisms, Phytophthora parasitica and Phytophthora citrophthora. Symptoms include rotted roots; cracked bark, accompanied by gumming (photo 1); water-soaked, reddish-brown to black bark at the soil line; discolored tissue in the lower trunk; yellowing, sparse foliage and death of the tree (photo 2).
Phytophthora parasitica and Phytophthora citrophthora are fungal-like organisms that complete their life cycles in the soil. Phytophthora parasitica is most active at higher temperatures (85-90F) and is prevalent at lower elevations, while Phytophthora citrophthora has a lower optimum temperature for growth (75-80F). In wet soils, both of these organisms produce motile spores, known as zoospores, which are attracted to host roots where they germinate and infect root tissue.
Phytophthora foot rot is controlled by use of resistant rootstock, improving drainage, and timely applications of effective fungicides. Excess soil moisture favors production and spread of zoospores. Therefore, increased drainage and careful irrigation is an important factor in disease management. Resistant rootstocks are available and should always be considered when choosing planting material. Moderately resistant rootstocks include Citrus macrophylla, Cleopatra mandarin and Troyer citrange. Effective chemicals registered for use on citrus include Ridomil Gold and Aliette, but there are restrictions for application. After a confirmed diagnosis, citrus should be treated according to label instructions.
A more detailed publication about citrus diseases is available from the College of Agriculture and Life Sciences or on-line at http://Ag.Arizona.Edu/PLP/plpext/publication/publication.html.
For more information see the following web sites:
http://www.ipm.ucdavis.edu/PMG/r107100411.html
http://www.ipm.ucdavis.edu/PMG/r107100111.html
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February 7, 2008