June 2002

Studying Phytophthora : Foes of Floral Crops By Margery Daughtrey

Phytophthora is one of the special group of plant pathogenstermed “oomycetes” — Pythium and the downy mildews are close kin.Analysis of their DNA has indicated that these organisms are more closelyrelated to algae than to fungi. This is the reason why Phytophthora, Pythiumand downy mildews are successfully combated by unique fungicides, such asmefenoxam, metalaxyl and dimethomorph, that would not be effective against thetrue fungi. For convenience, we’ll still refer to Phytophthora as a”water mold”; this is a useful term for remembering that wetconditions favor its nefarious activities against plants.

Existing and expanding enemies

Several Phytophthora species are fairly familiar foes,having caused a lot of problems on azaleas, rhododendrons, fuchsia, gloxiniaand poinsettia over the years. However, new production systems featuringsubirrigation — and recirculating systems, in particular — haveprovided a new playing field slanted towards this pathogen. New species ofPhytophthora are also getting into the act of attacking plants and causing newheadaches for growers.

Previous Phytophthora problems on poinsettias have primarilycome from Phytophthora parasitica, a species that thrives on warm, moistconditions. This is the same species that commonly causes symptoms on fuchsiasand gloxinias. Stem discoloration and wilting typify the attack on poinsettias,whereas gloxinias show a crown rot that bleeds into the base of the leaves, andfuchsias contract a stem rot that also bleeds into the leaf bases.

Now there are several new Phytophthora threats of concern toflower growers. The late blight disease, caused by Phytophthora infestans andnotorious for its effect on potatoes, occurs occasionally on both petunias andtomato transplants. For poinsettias, there is a new problem with the speciesPhytophthora drechsleri, which causes a root and crown rot. A third new Phytophthoradisease has also begun to appear on calibrachoa, causing wilting and death ofplants.

Phytophthora drechsleri on poinsettias

P. drechsleri on poinsettias has become a significantproblem. This disease causes the most extensive losses in poinsettias beingpropagated in ebb-and-flood irrigation systems. Because the pathogen producessporangia, which produce swimming spores that initiate new infections, theenvironment provided by a subirrigation system is perfect for distributinginoculum from plant to plant. P. drechsleri has a wide host range, so it islikely that it will appear on other flower crops if growers are not careful todisinfest areas where the disease has occurred on poinsettias. 

Fungicidal Control. SinceP. drechsleri is newly significant to the flower industry, our first effortshave been to determine whether the standard fungicides will indeed work. We arealso looking ahead to see if products still under development would be of useagainst the diseases that it causes. We conducted a trial in 2001 to comparesome familiar and some less familiar materials for their effect against P.drechsleri on poinsettia. We transplanted rooted cuttings of ‘FreedomMarble’ into ProMix BX on September 12, taking care to set the plants deeplyinto the mix to encourage a crown rot disease. There were five 3-plantreplications 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 addedafter the first application of the fungicide drenches by placing a disc ofcolonized agar into a depression about one inch away from each plant stem. Werecorded symptoms of wilting and cankers on November 6, and also rated the roothealth on a 1-4 scale (1=best roots, 4=complete root rot).

The results. Wiltingwas seen in two of the inoculated, untreated control plants on September 24. Atthe end of the trial, slightly over 50 percent of the plants in this untreatedgroup were wilting, and all of these showed a canker at the base of the stem.Several treatments with registered fungicides gave complete suppression of rootrot symptoms: Aliette 80WDG at 12.8 oz./100 gal., Truban 30WP at 6 oz./100gal., Banol 66.5 percent EC at 20 fl oz./100 gal. and Heritage 50WDG at 0.8oz./100 gal.

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., Biophos30.2L at 1 percent v:v and BAS 500 02 F 21.9 WP at 16 oz./100 gal. Theappearance of symptoms on some plants in the SubdueMAXX 21.3 percent ECtreatment (1.0 oz./100 gal.) suggests that there may have been some level ofinsensitivity to mefenoxam in the isolate of P. dreschleri that we used in thisstudy. Benefits similar to that seen with SubdueMAXX were seen in treatmentswith Hurricane 48WP and Medallion. It appears that a number of the currentlyregistered products, as well as some now in development, will be quiteeffective for managing P. dreschleri root and crown rot in poinsettias.

Calibrachoa Corruption

For the past two years, the popular new hanging basket cropCalibrachoa has performed unevenly, with many growers reporting problems ofwilting and root rot. We have isolated a number of Pythium and Phytophthoraspecies from calibrachoa and are in the process of identifying these anddetermining their pathogenicity. In one 2001 trial, we tested some of thestandard water mold controls (Subdue MAXX, Truban, Aliette and Banol) using sixplants per treatment in five replications. Two months after inoculation with anunidentified species of Phytophthora, symptoms were dramatic in the untreatedCalibrachoas — 70 percent of these plants wilted and died. All of thefungicide treatments (applied at a 1-month interval) reduced the symptomssignificantly. SubdueMAXX 21.3 percent EC at 1.0 fl.oz./100 gal. and Truban 30Wat 6 oz./100 gal. were slightly more effective than the Aliette 80WDG at 12.8oz./100; Banol 66.5 percent EC at 20 fl. oz./100 gal. was intermediate at thelevel of control provided.

What should the grower do?

Growers should stay informed regarding which of their cropsare prone to Phytophthora diseases and scout them especially carefully for anyroot 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 rotsymptoms on plants early in the production season. Fungicides can be effectivefor management, but these work by protecting plants against infection, soprompt use is essential. Utilize materials in rotation: Sole use of a mefenoxamor metalaxyl material is particularly inadvisable, since isolates ofPhytophthora insensitive to this chemistry have been documented in flowerproduction greenhouses. Meanwhile, we will continue to study Phytophthoramanagement — the Floriculture and Nursery Research Initiative hasinvested in research at Cornell, Clemson, Michigan State and NC State to focuson this important need.