Two Phytophthora's To Worry
About
By
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.