Disinfecting Diseases

July 14, 2008 - 10:33

My research at Cornell’s Long Island Horticultural Research & Extension Center often focuses on oomycetes, because the diseases they cause are so ubiquitous in greenhouse flower culture. As part of my extension work, I sleuth out the causes for 400 to 500 diseases on woody and herbaceous ornamentals each year. In 2007, for example, there were 32 cases of Pythium root rot and 15 cases of Phytophthora root and crown rot on ornamental plants submitted by growers who were puzzled by poor growth of their crops. The symptoms of downy mildews also mystify growers, as these diseases may be hard to recognize, and the telltale sporulation may be quite sparse. Last year, we observed downy mildew diseases on coleus, agastache and bracteantha. In 2008, downy mildew problems have already been on coleus, Jamesbrittenia and rose plant material shipped into New York.

With the aid of various specialized culturing techniques — and the essential assistance of my hardworking staff, Maria Tobiasz, Jadwiga Komorowska-Jedrys and Genevieve Giroux — I am able to assist growers in identifying the species of Pythium and Phytophthora assailing their crops. The Pythium and Phytophthora isolates are obtained in water cultures or on solid agar media. Sometimes the Pythium strains are grown on sterilized grass blades floating in water to coax them to produce their characteristic spore structures. Even light is manipulated to encourage Pythium and Phytophthora species to produce the type of spore desired: Sexual spore structures, not surprisingly, are best produced in the dark.

Recent isolations from greenhouse and nursery crops have included Pythium irregulare from bellis, brunnera, dahlia, geranium, lamium, lavender, oxalis and poinsettia. Numerous cases of Pythium aphanidermatum root and crown rot have occurred on calibrachoa, chrysanthemum and poinsettia during the warmest part of the year. These two species have been the main Pythium pathogens obtained from clinic samples on Long Island ever since identifications have been made to the species level.

Root Rot and Bulb Disease Prevention

One of my primary research projects is a large collaborative effort with Gary Moorman at Penn State, Steve Jeffers at Clemson, Gary Chastagner at Washington State University, Steve Wraight at USDA-ARS and Bill Miller in the Cornell Department of Horticulture. We have banded together to study root and bulb rot disease prevention, with a major focus on diseases caused by oomycetes. I also collaborate closely with Wraight and John Sanderson of the Cornell department of entomology on studies of fungus gnat interactions with root disease. These multidisciplinary, multistate research efforts are funded through the USDA-ARS Floriculture and Nursery Crop Research Initiative.

A number of greenhouse operations on Long Island work with my lab, giving us free rein to take regular samples from greenhouse floors, growing crops and media that are critical to our Pythium-tracking research. We conduct experiments at the LIHREC evaluating both biological and chemical controls for Pythium species. Experimental and registered fungicides are trialed for effectiveness in different host-pathogen systems. Three species, Pythium irregulare, P. ultimum and P. aphanidermatum are used in most of the trials, as these are the three species found most often in samples collected in flower production greenhouses.

One important recent finding from Dr. Moorman’s lab was that Pythium irregulare is actually two separate species with indistinguishable physical characteristics. The new species he identified, P. cryptoirregulare, is often found to be resistant to mefenoxam (found in SubdueMAXX) while P. irregulare is actually susceptible to this fungicide.

Downy Mildews

Although they infect the above-ground parts of plants, downy mildews are close relatives of Pythium and Phytophthora species. These diseases are often quite host-specific but may cause extensive damage on a single susceptible flower crop in a season in which the environment favors disease. In recent years, our diagnostic lab has found downy mildew diseases on agastache, argyranthemum, aster, coleus, creeping phlox, geranium, geum, helichrysum, hellebore, iberis, impatiens, pansy, rose, rudbeckia, salvia, snapdragon and veronica.

Many of these diseases, such as those on snapdragon and rose, have long been known, but new diseases in this category are appearing with surprising frequency in the past decade. Although reported previously from wild impatiens species in the United States, the downy mildew on Impatiens wallerana bedding plants appeared for the first time in the United Kingdom in 2003, and was seen on greenhouse crops in the United States a year later. The argyranthemum downy mildew, Peronospora radii, was observed in the United States for the first time in 2003, when it began to be a problem on California crops of Marguerite daisy. The same species, Argyranthemum frutescens, is also popular in the bedding plant trade, where it is just as susceptible to this downy mildew as it is in field production in California.

Coleus Downy Mildew

The most recent downy mildew to crash into the greenhouse flower trade is the Peronospora sp. that attacks coleus. This disease was unknown prior to 2005, when it was first noted on coleus in greenhouses and landscapes in Louisiana and New York. Gordon Holcomb at LSU and Brian Eshenaur of Cornell Cooperative Extension helped me document the first appearances of the disease in the United States. Subsequently, I have joined forces with Mary Hausbeck at Michigan State University to develop critical information on coleus downy mildew management with funding from the American Floral Endowment.

Host-range trials at the LIHREC have shown that the coleus downy mildew may affect basil, perilla and agastache as well as coleus. Collaborations with Lassaad Belbahri at the School of Engineering of Lullier, at the University of Applied Sciences of Western Switzerland, have indicated that the new coleus downy mildew is closely related to one causing disease losses of greenhouse basil crops in Italy and Switzerland.

At the LIHREC, our tests of coleus cultivars in greenhouse and landscape have shown that virtually all coleus cultivars are susceptible — more than 80 have been shown to be able to contract downy mildew thus far. In a trial last summer, all of the 49 cultivars tested were found to be susceptible to the disease: Sporulation on the undersurface of the leaves occurred on all plants (detected by microscopic observation) by the end of August. On some cultivars — ‘Gay’s Delight’, ‘Gold Lace’, ‘Mrs. Harding’ and ‘Solar Furnace’ — the leaves with sporulation showed no obvious symptoms of injury. These are examples of potential “Typhoid Marys” for coleus crops. Other plants, such as the cultivars ‘Florida City Chuluota’, ‘Violet Tricolor’, ‘Solar Storm’ and ‘Pistachio Nightmare’, showed symptoms clearly on the foliage: both leaf spotting and leaf curl.

Although we observed variation in symptoms, there were no resistant cultivars among those trialed. The variation in how coleus cultivars respond to downy mildew will be key to developing a long-term solution of the problem. Ultimately, it appears that growers will elect to grow only coleus cultivars with lower susceptibility to the disease, those that will not exhibit the severe leaf curling and leaf drop shown by some popular cultivars today when they are infected with downy mildew. The plants with chartreuse foliage are often extremely susceptible to the disease, so identifying coleus in this category with more resistance will be important.

Fungicides for Coleus Downy Mildew

Our trials of fungicide performance against the coleus downy mildew have shown interesting results. Some of the treatments that are quite effective against other downy mildew diseases are not strong performers when it comes to reducing the coleus disease. For example, this particular downy mildew does not appear to be vulnerable to some of the strobilurin fungicides (Heritage and Compass) or to the phosphorous acid materials tested (Aliette and Alude).

The best preventive activity against coleus downy mildew infection in our trials to date has been seen with Stature DM and Fenstop treatments, and with mancozeb. Mancozeb fungicides (e.g., Protect DF) will give only contact action, but they can be effective against the downy mildew as protectants if coverage is good. If SubdueMAXX drenches are used for Pythium control, these will also help to curtail downy mildew infections on coleus.

As yet, it is still unclear whether the seed of coleus may carry downy mildew infection through to young plants. Plants in plug trays have been found to be heavily infected with downy mildew, but they have always been produced on premises where vegetatively propagated coleus were also being grown. Seed harvested from diseased coleus planted at the LIHREC greenhouses has not given rise to infected seedlings.

The exact temperature, day length and humidity necessary for downy mildew infection and sporulation on coleus is not yet known, but it is already apparent that the disease is favored by warmer conditions than many of the other downy mildew diseases. Outbreaks in northern U.S. greenhouses in the spring often follow the introduction of plants produced in the South under different, presumably warmer, environmental conditions.

Other Research Efforts

Of course, other groups of pathogens demand attention as well: viruses, bacteria, nematodes and fungi all vie with oomycetes for research and extension attention. Powdery mildew of verbena has been the subject of several comparative trials that have identified a wide range of susceptibility in cultivars. New fungicides and biocontrols are also pitted against powdery mildew fungi affecting gerbera, rose and poinsettia to determine their range of effectiveness. Black root rot (with culprit Thielaviopsis basicola) management studies have been another focus; the importance of this fungus has recently increased because reuse of trays, flats and pots has become a widely adopted cost-saving measure.

Please see the 2007 LIHREC Annual Report (www.hort.cornell.edu/department/ facilities/lihrec/2007report.htm) for summaries of our recent trials on diseases of ornamentals. Visuals to help with identification of some diseases may also be found at the Pest and Diseases Photo Gallery, www.greenhouse.cornell.edu.

About The Author

Margery Daughtrey is senior extension associate in the department of plant pathology and plant-microbe biology at Cornell University’s Long Island Horticultural Research & Extension Center. She can be reached at mld9@cornell.edu.

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