Phytophthora is one of the special group of plant pathogens
termed “oomycetes” — Pythium and the downy mildews are close kin.
Analysis of their DNA has indicated that these organisms are more closely
related to algae than to fungi. This is the reason why Phytophthora, Pythium
and downy mildews are successfully combated by unique fungicides, such as
mefenoxam, metalaxyl and dimethomorph, that would not be effective against the
true fungi. For convenience, we’ll still refer to Phytophthora as a
“water mold”; this is a useful term for remembering that wet
conditions favor its nefarious activities against plants.
Several Phytophthora species are fairly familiar foes,
having caused a lot of problems on azaleas, rhododendrons, fuchsia, gloxinia
and poinsettia over the years. However, new production systems featuring
subirrigation — and recirculating systems, in particular — have
provided a new playing field slanted towards this pathogen. New species of
Phytophthora are also getting into the act of attacking plants and causing new
headaches for growers.
Previous Phytophthora problems on poinsettias have primarily
come from Phytophthora parasitica, a species that thrives on warm, moist
conditions. This is the same species that commonly causes symptoms on fuchsias
and gloxinias. Stem discoloration and wilting typify the attack on poinsettias,
whereas gloxinias show a crown rot that bleeds into the base of the leaves, and
fuchsias contract a stem rot that also bleeds into the leaf bases.
Now there are several new Phytophthora threats of concern to
flower growers. The late blight disease, caused by Phytophthora infestans and
notorious for its effect on potatoes, occurs occasionally on both petunias and
tomato transplants. For poinsettias, there is a new problem with the species
Phytophthora drechsleri, which causes a root and crown rot. A third new Phytophthora
disease has also begun to appear on calibrachoa, causing wilting and death of
P. drechsleri on poinsettias has become a significant
problem. This disease causes the most extensive losses in poinsettias being
propagated in ebb-and-flood irrigation systems. Because the pathogen produces
sporangia, which produce swimming spores that initiate new infections, the
environment provided by a subirrigation system is perfect for distributing
inoculum from plant to plant. P. drechsleri has a wide host range, so it is
likely that it will appear on other flower crops if growers are not careful to
disinfest areas where the disease has occurred on poinsettias. style="mso-spacerun: yes">
Fungicidal Control. Since
P. drechsleri is newly significant to the flower industry, our first efforts
have been to determine whether the standard fungicides will indeed work. We are
also looking ahead to see if products still under development would be of use
against the diseases that it causes. We conducted a trial in 2001 to compare
some familiar and some less familiar materials for their effect against P.
drechsleri on poinsettia. We transplanted rooted cuttings of ‘Freedom
Marble’ into ProMix BX on September 12, taking care to set the plants deeply
into the mix to encourage a crown rot disease. There were five 3-plant
replications for each of 15 treatments in a randomized, complete block design.
Treatments with Biophos, Banol and BAS 500 02 F were made on a 21-day interval,
whereas others were repeated after 28 days. Inoculum of P. dreschleri was added
after the first application of the fungicide drenches by placing a disc of
colonized agar into a depression about one inch away from each plant stem. We
recorded symptoms of wilting and cankers on November 6, and also rated the root
health on a 1-4 scale (1=best roots, 4=complete root rot).
The results. Wilting
was seen in two of the inoculated, untreated control plants on September 24. At
the end of the trial, slightly over 50 percent of the plants in this untreated
group were wilting, and all of these showed a canker at the base of the stem.
Several treatments with registered fungicides gave complete suppression of root
rot symptoms: Aliette 80WDG at 12.8 oz./100 gal., Truban 30WP at 6 oz./100
gal., Banol 66.5 percent EC at 20 fl oz./100 gal. and Heritage 50WDG at 0.8
Several experimentals also completely suppressed symptoms:
Ardent 50WP at 0.6 and 0.8 lb./100 gal., EXP 10623 A at 7 oz./100 gal., Biophos
30.2L at 1 percent v:v and BAS 500 02 F 21.9 WP at 16 oz./100 gal. The
appearance of symptoms on some plants in the SubdueMAXX 21.3 percent EC
treatment (1.0 oz./100 gal.) suggests that there may have been some level of
insensitivity to mefenoxam in the isolate of P. dreschleri that we used in this
study. Benefits similar to that seen with SubdueMAXX were seen in treatments
with Hurricane 48WP and Medallion. It appears that a number of the currently
registered products, as well as some now in development, will be quite
effective for managing P. dreschleri root and crown rot in poinsettias.
For the past two years, the popular new hanging basket crop
Calibrachoa has performed unevenly, with many growers reporting problems of
wilting and root rot. We have isolated a number of Pythium and Phytophthora
species from calibrachoa and are in the process of identifying these and
determining their pathogenicity. In one 2001 trial, we tested some of the
standard water mold controls (Subdue MAXX, Truban, Aliette and Banol) using six
plants per treatment in five replications. Two months after inoculation with an
unidentified species of Phytophthora, symptoms were dramatic in the untreated
Calibrachoas — 70 percent of these plants wilted and died. All of the
fungicide treatments (applied at a 1-month interval) reduced the symptoms
significantly. SubdueMAXX 21.3 percent EC at 1.0 fl.oz./100 gal. and Truban 30W
at 6 oz./100 gal. were slightly more effective than the Aliette 80WDG at 12.8
oz./100; Banol 66.5 percent EC at 20 fl. oz./100 gal. was intermediate at the
level of control provided.
Growers should stay informed regarding which of their crops
are prone to Phytophthora diseases and scout them especially carefully for any
root rot or wilt symptoms. Examine the roots of all incoming plant material,
and use a diagnostic laboratory to identify the agents of any root or stem rot
symptoms on plants early in the production season. Fungicides can be effective
for management, but these work by protecting plants against infection, so
prompt use is essential. Utilize materials in rotation: Sole use of a mefenoxam
or metalaxyl material is particularly inadvisable, since isolates of
Phytophthora insensitive to this chemistry have been documented in flower
production greenhouses. Meanwhile, we will continue to study Phytophthora
management — the Floriculture and Nursery Research Initiative has
invested in research at Cornell, Clemson, Michigan State and NC State to focus
on this important need.
Trials conducted at Cornell University show which fungicides effectively controlled Phytophthora species on both poinsettia and calibrachoa crops.