Two Phytophthora's to Worry About
By Nick Sakovich
Growers have been wrestling with root problems, like phytophthora root rot, for a long time. But in citrus, we have a slight variable. We are dealing with two phytophthoras - a summer and a winter - or warm season and cool season - species. The damage is similar, but for treatment, it is essential to know which species is doing the damage.
In trees infected with phytopythora root rot, the feeder roots are destroyed, causing a severe limitation in water and nutrient uptake. This results in poor growth, small fruit size and lower yields. If the infection takes place during the cool winter or spring, typically a time of low water demand (low ET), symptoms may not readily be evident. However, once the summer heat arrives, with its high water demand, these trees whose feeder roots have been destroyed, will readily show symptoms of poor growth, yellow leaves and leaf drop. The symptoms are seen in the summer, but the damage had been done perhaps several months prior.
Phytophthora citrophthora prefers the cooler temperatures and is most active from November to April. In addition to damaging feeder roots, it also causes brown rot of the fruit and gummosis. Phytophthora parasitica prefers the warmer temperatures. It has an optimum growing temperature near 90 degrees Fahrenheit, and is active from June to November. These active periods will of course vary somewhat due to location and climate. P. parasitica can also cause gummosis and brown rot as long as other necessary conditions are present. For instance, during the summer, if the fungus is present and there is ample moisture, such as rain or irrigation water contacting the trunks of susceptable trees, gummosis can result.
For control of phytophthora root rot and gummosis, the first step is the use of resistant rootstocks followed by proper water management. (This has been discussed in past California Grower articles) If necessary, chemical control should be implemented. Aliette and Ridomil are two fungicides registered for the control of phytophthora root rot and gummosis in citrus. Timing is critical for root rot control.
Sampling for P. citrophthora should be done January through March; P. parasitica, July through September. In treating for P.citrophthora, the best time would be November/December and February/March; for P. Parasitica, June/July and August/September.
Knowing that the trees are infected with phytophthora is important, because, for one thing, the grower can now look more closely at his irrigation scheduling. But if chemical treatment is called for, you must know which phytophthora is infecting your orchard so that proper timing of the fungicide can be ensured.