PSI: The case of the Soft Rots

The Rundown

Who + How: Erwinia (now hiding under several aliases, including Pectobacterium and Dickeya) is a one-celled plant enemy. Alone and single, it’s no threat… but when it divides and multiplies at a prodigious rate, as it does in a warm, moist greenhouse, it produces millions of cells per day. Once inside the plant, each cell puts out minuscule drops of enzymes that turn leaves, stems and roots to mush. The name “Pectobacterium” is a hint that this critter goes for pectin — pectin holds the jelly together, and it also holds plant cells together. We call this sort of bacterial disease “soft rot” because the attacked tissue disintegrates.

What: Bacterial soft rots can affect many plants. Foliage plant growers are the most likely to see them. There is also soft rot bacteria that fiendishly attacks callas, and some that rot poinsettia cuttings or wilt entire plants in midsummer. When these bacteria get into cyclamen corms, it gives them the texture of baked Brie. The bacteria can be moved about on your hands, by insects, or by splashing of irrigation water. They can ride along on the surfaces of plants brought into the greenhouse.

Where: It’s everywhere! It’s everywhere! The word ubiquitous applies to soft rotters. Your greenhouse may not harbor all the different kinds of soft rot bacteria, but chances are there are some present. Envision these tiny cells on the floor, benches and plants and then undertake regular sanitation efforts to keep the bacterial population down.

When: Soft rot bacteria are fond of warm, steamy conditions and they will be most aggressive in areas and during seasons in which temperatures are in the 80s and 90s. They are also the couch potatoes of the pathogen world, lazily taking advantage of open wounds (at the base of cuttings) or natural openings, like stomata or hydathodes — given an easy entrance, soft rots move into plants and feed on them, with ugly symptoms as a result. The soft rot bacteria secrete plant-digesting enzymes, just like a Venus fly trap secretes insect-digesting enzymes — unglued, collapsed cells are left in their wake.
 

Monitoring

Unless you have a cold, it’s easy to know when a plant has bacterial soft rot: it often smells like stinky, rotten fish. On leaves, look for dark, water-soaked areas. These will dry to a tan color when humidity is low, but reactivate and enlarge during another wet period. If wilted plants have a corm or rhizome, cut into it and see if it has a mushy texture. Stems affected by soft rot are slimy and sticky to the touch. Remember to be especially vigilant for soft rot when it’s hot and humid.
 

Prevention

Bacterial soft rot is best managed by sanitation. Using a disinfectant to clean working and growing surfaces is important. Any reused pots should have remnants of mix rinsed or brushed away before soaking in disinfectant. Try to control winged insect pests to keep them from moving bacteria around the greenhouse. Avoid overly deep planting, as this may encourage bacterial invasion of corms, rhizomes, etc. Wash your hands after handling any plants that appear to be diseased.
 

Treatment

• Discard infected plants because otherwise, bacteria will be splashed from them to their brethren at every irrigation.
• Reduce splashing: irrigate gently and carefully, and only when needed.
• In hot, humid weather conditions, treat soft-rot prone plants with copper or copper-plus-mancozeb bactericides.
• Rotate copper bactericides with biocontrols that have Bacillus subtilis as their active ingredient. Occasional treatments with quaternary ammonium or peroxide-based materials labeled for application to plants will also help to reduce bacterial populations, but note that they will not have a long residual.

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