PSI: The case of the damaging fungi

The Breakdown

What we call “Fusarium” is really many different diseases clumped under the name of a fungal genus. Both systemic vascular wilt diseases (Fusarium wilts) and a miscellaneous assortment of root and stem infections — even some leaf spots — are caused by Fusarium fungi. The root and stem problems are more common but less serious than the wilts. Fusarium diseases can have lethal consequences under conditions that favor their attack.

The host-specialized Fusarium wilt fungi each affect only one crop, or very closely related plants. Aster, basil, begonia, carnation, chrysanthemum, cyclamen, dahlia, gerbera, gladiolus, lily, lisianthus, Marguerite daisy, exacum, ranunculus and tulip all have a Fusarium wilt disease. Propagators make heroic efforts to keep Fusarium out of the chain of production with their culture-indexing, monitoring and sanitation measures. Cyclamen is especially hard to produce free from its Fusarium wilt because the pathogen is seedborne. Spread of Fusarium wilt fungi is most common through vegetative propagation. The more omnivorous root and stem rot Fusarium pathogens may be spread by movement of invisibly-infected plants or by soil contamination of a growing mix (by workers or insects).

The species of Fusarium that cause root rots and stem rots (such as Fusarium solani or F. avenaceum) are common soil dwellers, and tend to have wide host ranges. The Fusarium wilt pathogens, on the other hand, are closely associated with the crops they can parasitize, and are more likely to arrive as latent infections within the xylem of cuttings rather than being blown or splashed in from the environment. Once Fusarium wilt has occurred in your greenhouse the pathogen is very talented at surviving in crop debris or soil on the floor — so the threat of disease can continue if there are no effective sanitation efforts.

Fusarium root and stem rots can strike at any time. These attacks are helped by fungus gnats or cultural actions that injure roots or stems. Fusarium wilts often become more obvious during higher growing temperatures (summer).
 

Monitoring for disease

Check your crop’s root health regularly: irregular heights may indicate root rot or stem cankers.

Watch for lower leaf yellowing or wilting that might indicate Fusarium wilt in a crop known to be susceptible.

One-sided wilting or a dark discoloration of the vascular system are clues that may indicate a Fusarium wilt disease.

A lab diagnosis will help you to know which kind of disease you are fighting.

Monitor fungus gnats, which may spread Fusarium species, using yellow sticky cards (adults) or potato chunks (larvae).

Prevention of disease

Have impeccable sanitation in your greenhouse, cleaning especially after a disease outbreak.

Avoid overfertilization or overwatering, which will injure roots and provide entry for Fusarium fungi that cause root and stem rot.

Manage fungus gnat populations that could aggravate Fusarium problems.

Order from the most reliable suppliers when purchasing plants susceptible to a Fusarium wilt disease: purchase clean stock for any crop for which it is available.
 

Treatment of disease when present

Dispose of diseased plants promptly.

Chemical control is often inadequate, especially for the wilts. Materials that suppress Fusarium somewhat include azoxystrobin, fludioxonil, iprodione, pyraclostrobin and thiophanate-methyl, as well as some biological controls.

For vascular wilts, try growing at a high pH (6.2 or above) and with nitrate rather than ammonium forms of nitrogen. This cultural scenario strongly suppresses Fusarium.

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.