Pythium root rot is familiar to all greenhouse growers — it shows up on everything from poinsettias to microgreens. You probably know benches and flats need to be very clean, and that you need a well-drained mix to keep this disease from pouncing. But did you know that the Boogeymen causing Pythium root rot have recently had name changes? Pythium root rot is now caused by things called “Globisporangium” and “Phytopythium” — don’t they run trippingly on the tongue? — and some are still known as “Pythium” (see Table). Don’t let this fool you — the diseases are the same, only the pathogen names have changed!

With funding from the Floriculture and Nursery Research Initiative, we have been collecting information on Pythium root rot pathogens for a number of years. Most recently, in 2017 we gathered 209 isolates from 1,080 samples taken from growing media from crops at seven businesses on Long Island. The pathogens were identified as Globisporangium irregulare, Pythium aphanidermatum and P. myriotylum. The largest number of root-rotter isolates was collected from dahlia and garden mums, with only a few each from other crops: angelonia, bacopa, caladium, calla, calibrachoa, colocasia, cyclamen, geranium, hibiscus, impatiens, petunia, poinsettia, salvia and snapdragon.

The dahlias often yielded Globisporangium species, which are favored by moderate temperatures (86° F for G. irregulare). The mums, in contrast, had Pythium aphanidermatum and P. myriotylum both of which are summer-season loving Pythiums, favored by 95-104° F and 98-104° F, respectively (but both species can tolerate temperatures below 86 and above 104° F). We found these agents of Pythium root rot in pots containing either healthy or sickly looking plants. Our sampling has convinced us that it is not simply the presence of a Pythium or Globisporangium species that results in disease — it’s so often the environmental conditions that determine whether symptoms appear.

Pythium aphanidermatum on holiday cactus
Photo: Margery Daughtrey

Identifying these pathogens to species is just a first step in our studies. Samples of the mycelium of the isolates are frozen and shipped to Oklahoma, where molecular diagnostic techniques are applied. Using these techniques, the Globisporangium we collect in New York can be separated into two almost identical looking species, G. irregulare and G. cryptoirregulare. These two organisms may take on different roles in the greenhouse: G. cryptoirregulare is a particular foe of New Guinea impatiens and it has also been found associated with root rot on geranium, gerbera and salvia. G. irregulare has been isolated from a variety of plants with root rot, including geranium, gerbera, ranunculus and lupine. In the past the Pythium aphanidermatum we have collected has seemed very uniform, almost clonal. We are pursuing the hypothesis that further molecular analysis, using microsatellite analysis and genotyping by sequencing, may allow us to see differences between P. aphanidermatum afflicting mums versus strains on poinsettias.

Management considerations

Ideally, we would be producing only naturally disease-resistant plants. The cultivars prone to Pythium root rot, however, are usually mixed in with those with low susceptibility, and it is rare that growers can find information on which cultivars to grow to avoid root rot. Researchers have sometimes gathered information to help growers with cultivar choices. Zhanao Deng at the University of Florida, for example, compared important caladium cultivars and found that some were preferable to others with regard to their Pythium myriotylum susceptibility. In his trials, one cultivar was super-susceptible and had 85 percent root rot. Seven cultivars had 35-94 percent root rot, while eight lost only 15-35 percent of their roots to Pythium. Four of the caladiums tested, however, had moderate levels of resistance — only 5-14 percent of their roots rotted: Freida Hemple, Candidum, Candidum Jr. and White Christmas. The first step in IPM should be to choose to grow the plants that are naturally less disease-prone. Even without the benefit of published data, you can note for yourself when cultivars appear to be prone to root rot, and aim to find substitutes.

There are several potential sources for the agents of Pythium root rot. The greenhouse may harbor inoculum from earlier infestations in organic debris. Reusing trays or flats from an earlier crop can bring Pythium root rot back. Seeds, or organic debris mixed with seeds, could bring inoculum to a plug tray. And contaminated or infected but asymptomatic cuttings could occasionally bring Pythium over from another greenhouse, even from another country. Symptoms may be delayed until plants are under stress or until the environmental conditions are just right for disease development, so it is generally impossible to tell when or where a root rot outbreak originated. Your goal as a grower is to keep Pythium root rot pathogens out of the greenhouse (keep field soil out and discard infected plants) and then to grow (and ship) the crop under non-stressful conditions.

Globisporangium irregulare on verbena
Photo: Margery Daughtrey

Poorly drained growing mix is one invitation to Pythium root rot, and excess nitrogen is another, as Gary Moorman of Penn State University showed in his research on geranium. Letting the crop dry down too far before watering is the kind of accident that won’t necessarily kill the plants, but may lead to infection by Pythium because of the root injury.

Your defenses against Pythium root rot include biological and chemical controls. Biocontrols that are antagonistic to Pythium and Globisporangium spp. include species of Trichoderma, Bacillus, Streptomyces and Pseudomonas. This is fighting fire with fire, pitting one microorganism against another. The beneficial biological agents occupy ecological niches that could otherwise be filled by pathogens, compete with the pathogens for nutrients, and inhibit their growth. For best results, use these friendly microbes from the beginning of the crop, in the plug tray or at transplant, so that they can establish themselves on the root system and help to fend off pathogens.

The chemicals that work against Pythium root rot form a fairly short list. The labeled materials in the U.S. include mefenoxam, strobilurins, etridiazole, cyazofamid, phosphorous acids and fluopicolide. Mefenoxam has been found to have significant problems with resistance: It may not control Pythium at all in some situations, and may even stimulate resistant strains at subtoxic doses, resulting in higher disease frequency and severity. Rotating among fungicides with different modes of action is fundamental to avoid resistance development. The strobilurins are not all labeled for Pythium control, and they generally work far better against Phytophthora than against Pythium root rot. Follow all label instructions to avoid encouraging resistance or causing phytotoxicity.

Growers who are good at integrating all these considerations — sanitation, cultural, biological and chemical control measures — will only rarely be troubled by Pythium root rots, no matter what name is associated with the pathogen!

Carla is an associate professor of plant pathology at Oklahoma State University researching soil-borne diseases. She aims to help growers optimize the use of disease management resources.  carla.garzon@okstate.edu 

Margery is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases. mld9@cornell.edu