Minding water molds

Phytophthora can be a root rot and a foliar blight. On this flat of vinca, it is both.

Greenhouse growers face a diversity of plant pathogens, most of which are barely visible to the naked eye. The destruction they cause is obvious, not to mention costly. Understanding what these disease-causing organisms are and how they live is essential to managing them and limiting their destructive potential — before you ever see them or the damage they inflict.

Water molds are quite possibly one of the most destructive groups of plant pathogens. At first glance, they seem very similar to fungi, and they share a lot of traits in common. Both are barely visible, spreading by fine threads called hyphae, and both produce unbelievable numbers of spores. But that is where their likeness ends. Water molds are more like algae than fungi, so the fungicides that control them aren’t the same as what you would use for Fusarium wilt or powdery mildews. The key take-home here is that many fungicides that work great on true fungi, like Cleary’s 3336 or Systhane, don’t work on water molds. Some of the most destructive water molds include downy mildews, Pythium and Phytophthora.

Water mold lifecycle
All of the water molds have a similar lifecycle: Upon germination from thick -walled oospores, hyphae emerge to directly infect or develop into a zoosporangium, a big word that describes a swollen sac that develops at the end of a hyphal thread and releases tiny, swimming zoospores in the presence of water. These zoospores then swim to and infect plants. The zoosporangium can also germinate and infect plants directly. Upon infection, new hyphae grow into and throughout the plant, absorbing nutrients as a food source and breaking down plant tissues. These water molds then form news zoosporangia or oospores to repeat the cycle.

One of the most important aspects to successfully managing water molds is to manage your water. This is true for all three pathogens. Avoid overhead watering, watering late in the day (which prevents leaves from drying) and poorly draining media. The floors and walks between beds should be free of puddles and weeds (they can serve as disease reservoirs) to reduce sporulation and spread of the pathogens, as in the case of Pythium and Phytophthora. Avoid using runoff water or pond or recycled water for irrigation. If using pond or stream water for irrigation, place the intake pipe well above the bottom of the pond so that sediment is not drawn in, but also make sure the pipe isn’t near the surface either. Slow sand filtration is an effective method for removing Pythium and Phytophthora from recycled water. Other water treatment options include chlorination, ultraviolet radiation and ozonation.

Sanitation is important, too. Propagation benches should be raised, and they should provide excellent drainage. All cleaned surfaces, propagating knives and tools should be washed with sanitizing agents (Physan 20, Zerotol or Greenshield (PT-2000)). Watering hoses must be kept off the floor to avoid the potential spread of these soil-borne pathogens onto the bench with the hose.

Downy mildew
Downy mildews can cause major damage to greenhouse crops in the form of leaf spots, blights, defoliation and distortion. Although there are many different downy mildews, the impatiens downy mildew is a new threat to flowering impatiens in greenhouses and landscape. Impatiens walleriana (including doubles and minis), I. walleriana hybrids (I. walleriana x auricoma and I. niamniamensis x walleriana) and garden balsam (I. balsamina) are susceptible. New Guinea impatiens (I. hawkeri) are extremely tolerant to this disease, meaning that the infected plants do not show obvious symptoms, even under severe disease pressure. Although impatiens downy mildew is new, different species of downy mildew also infect other crops, including pansy, alyssum and osteospermum, and mint family members including coleus, salvia, rosemary, lamium and basil.

Early symptoms of downy mildew on any crop are difficult to detect. Leaves may be slightly yellow (chlorotic) with stippling and may mimic nutritional deficiency or spider mite damage. Leaves droop and the plants take on an “unthrifty” appearance. Under cool, wet humid conditions, white-colored sporulation may be visible on the undersides of the leaves, but it is important to stress that sporulation may not always be visible, especially if conditions are warm and dry, or if fungicides were applied curatively. As the disease progresses, plants appear stunted and foliage drops, resulting in green, leafless stems.

Downy mildew enters a greenhouse in one of two ways — via infected plugs or cuttings or by entry of wind-borne spores. One of the easiest ways to avoid this disease is to grow crops from seeds instead of cuttings. Although not foolproof (the pathogen could contaminate seeds if the grower harvests seeds from infected plants), several large growers have avoided impatiens downy mildew in their greenhouses through seed impatiens production instead of cuttings. Scouting to quickly identify and eradicate any newly infected plants is essential. Fungicides provide much better prevention of this disease, as control is all but impossible once the symptoms appear.

Pythium root rot
Pythium usually attacks plant roots, but it can also cause cutting rots, stem rots and foliar blight under the right conditions. It is a significant problem in the greenhouse industry. The first symptoms of Pythium infection often include stunting, with wilting occurring in the most severe cases. Over time, roots appear water-soaked and the root cortex easily sloughs off, leaving a strand of vascular tissue. On the stems of cuttings, a soft, watery rot may develop.

Pythium causes a symptom described as “black leg” on geranium when the pathogen attacks the stem starting at the soil line.

All Pythium species favor wet conditions with high soil-soluble salts in the potting medium. Pythium has been found contaminating commercially available soilless potting mixes, and it readily contaminates sterilized soil or soilless mixes by the careless use of dirty tools, containers and proximity to previously infected plants or media. Poorly draining medium or the placement of pots or flats in standing water will also affect drainage and predispose plants to infection by Pythium. Excess watering also creates conditions conducive for shore flies and fungus gnats, which feed on roots and damage them, providing an infection court for Pythium. These insects are also effective vectors of the pathogen, spreading the disease throughout the greenhouse or growing area.

If you have had problems in the past with Pythium, be proactive to prevent future outbreaks. Incorporate biological control agents that contain the “good” fungi Trichoderma harzianum or Gliocladium virens (the active fungi in Root Shield and Soil Guard, respectively) to provide some protection when disease pressures are low. Overwatering or excessive fertilization will reduce their efficacy, however, to the point where severe outbreaks of Pythium can occur despite the use of biological controls. If severe outbreaks have occurred in the past, consider incorporating a granular fungicide (for example, Banrot 8G) in your potting mix in lieu of a biological control agent, and re-evaluate your cultural practices that may result in excess water or fertilizer. Fungicide application for control of Pythium works best when applied as a preventive measure.

Phytophthora
Phytophthora species can cause root rots, crown rots and aerial blights. Sometimes the same pathogen can cause all three. There are many different species of Phytophthora, so only a few generalizations can be made about symptom development. In general, plants with mild root rot produce smaller-than-normal foliage; roots and crowns of these plants may reveal dead feeder roots and dark streaks up stem. Those plants with severe root rot often exhibit stunting of the entire plant. Foliage usually appears wilted, and leaves are smaller than normal. Petunias and calibrachoas in particular often appear to be suffering from iron chlorosis. In Vinca (Catharanthus), blight and root rot can be particularly devastating, particularly under warm wet conditions. They can wipe out flats in a matter of days.

The key to managing this disease is prevention. Remove and destroy infected plants. Avoid infection by using well-drained, slightly acidic planting media that are less conducive to Phytophthora than poorly drained media. When planting cuttings, do not plant too deeply, as this predisposes plants to infection. The use of mefenoxam (Subdue MAXX), fosetyl-Al (Aliette) or phosphorous acid (Biophos, Alude) is unlikely to kill the fungus in infected plants, but it can reduce the spread of Phytophthora throughout the site and protect healthy plants from infection. Remove severely infected plants prior to treatment.

Accurate diagnosis is essential before using any fungicide. For example, Stature DM is very effective against Phytophthora and downy mildew, but not Pythium. When developing a fungicide rotation, be sure to choose fungicides that have different FRAC codes to minimize the risk of fungicide resistance developing in your greenhouse.

As always, read and review labels. Labels, rates and usages often change depending upon which pathogen you are attempting to control and on which crop you are treating.

Janna L. Beckerman is associate professor of the department of botany and plant pathology at Purdue University. Her area of expertise is ornamental and fruit diseases.