Formidable Fusarium

Despite decades of research on all aspects Fusarium prevention and control, it remains one of the hardest pathogens to control. This is even true when plants are treated before infection. Fusarium can cause a multitude of symptoms from leaf spots, to cutting rot, crown rot, wilt, bulb rot and even root rot. Fusarium spores move easily with water and can be splashed from one plant to another. They often appear in propagation as cutting rot and are not always the primary pathogen involved. Wilt diseases are usually caused by Fusarium oxysporum with a separate type for each type of plant attacked. Leaf spots on Dracaena are caused by F. moniliforme and crown rot is often caused by F. solani.

Symptoms of Fusarium infection
A wide range of symptoms can be caused by Fusarium spp. Black or brown sunken lesions can form at the base of stems. There can be reddish streaking in petioles near crown and sometimes pinkish or white masses of mycelium all over cutting bases or crowns. These form most commonly when plants are in propagation and under mist or high humidity. Cutting rot and dieback as well as stem cankers with red or orange round fruiting bodies. These are the reproductive structures that can form in advanced infections. Stunting, wilting and yellowing are each signs of a Fusarium wilt infection.

Conditions that promote Fusarium
The most common way a Fusarium infection starts is through use of infected cuttings. Other factors that promote this disease are over-watering and use of poorly draining potting media. Dipping cuttings in a fungicide before sticking can actually spread spores very effectively making the entire batch contaminated. Planting in ground beds that have become contaminated from a previous crop only perpetuates the disease.

How does Fusarium spread?
Fusarium is a soil-borne fungus that usually makes spores (lots of them) that are somewhat sticky. They are easily spread by cutting instruments and water splashing from rainfall or overhead irrigation. One of the most effective ways to spread disease evenly through your cop is to dip un-rooted cuttings into a fungicide or PGR bath. The spores that might be contaminating less than 1 percent of the cuttings are then able to spread to all of the cuttings equally. The fungicide in the dip solution may help kill some of the Fusarium spores but more often than not, the fungicide chosen is not the most effective on this pathogen, the rate chosen is too low or the tank is kept too long. Once a surface is contaminated with Fusarium it is very hard to eradicate since this fungus is a good saprophyte. That means it does not have to live on a plant and not all Fusarium species are even plant pathogens.
 

Killing spores on surfaces
In some cases, Fusairum has been found contaminating the surface of seeds. One such case is Fusarium wilt of China aster (Callistephus chinensis). Studies conducted by Wade Elmer (Connecticut Agricultural Experiment Station) examined seed contamination, on-farm disease incidence (CT and FL), sanitation, host resistance, and various soil treatments. Results of seed testing showed 5 out of 25 commercial seed packages from three separate distribution companies had seeds contaminated with Fusarium oxysporum f. sp. callistephi. Pathogenic isolates from Florida and groups and may have had another origin. Sodium hypochlorite solutions (1 percent) eliminated the fungus from seeds and Styrofoam when applied as a soak or spray, respectively. The trials on Fusarium oxysporum fsp. callistephi on styrofoam disks are summarized in Table 2.

There are quite a few products being used to disinfest greenhouse surfaces like benches, floors and trays including chlorine products (like bleach and chlorine dioxide), peroxides (like X3 and ZeroTol), and quaternary ammoniums (like GreenShield, Physan and KleenGrow). We tested the efficacy of a few of these KleenGrow, GreenShield or ZeroTol. We tested efficacy on 4 ml polyethylene film, wood and concrete block with Fusarium oxysporum fsp. cyclamenis (the cause of Fusarium wilt on cyclamen). Surfaces were dirty when we started with obvious algae growth. The PAC was applied to each surface as a spray (5 oz/gal) and rinsed after 5 min. This was followed by one of the disinfestants – KleenGrow (1 oz/gal), GreenShield (1 tbsp/gal) and ZeroTol (1 percent). Recovery of Fusarium was attempted after 24 hours.

The percentage of Fusarium spores killed was affected by the type of surface. Plastic was the easiest surface to clean and wood the most difficult. PAC alone was somewhat effective. KleenGrow was very effective on all three surfaces when the PAC was used first with at least 93 percent kill. GreenShield was also very effective but was better without the PAC cleaning treatment. ZeroTol was least effective in this test with mixed results on the different surfaces.

Fungicides best for controlling Fusarium diseases (none of these are curative)

Some of the most effective fungicides for Fusarium have been identified through many research trials. Strobilurins like pyraclostrobin (Insignia or Pageant) or azoxystrobin (Heritage) are often some of the most effective. Others that are equally effective include: fludioxinil (Medallion), triflumizole (Terraguard) and iprodione (Chipco 26019). These can each be used for foliar, crown and root diseases casued by Fusarium. For foliar diseases only you can use chlorothalonil (Daconil) which remains one of the bets for Fusarium leaf spot on Dracaenas. In addition, many growers still use thiophanate methyl (3336, Fungo and OHP-6672).
 

There are a number of nursery practices that can make Fusarium diseases especially bad. Never re-use pots or flats without a thorough cleaning and disinfesting. Never re-use potting media. Do not use a single product without rotation – fungicide resistance is a serious problem in some cases. Do not guess what is wrong – many crown rot diseases look alike and if you treat for the wrong one your efforts will fail. Finally, do not use fungicide dips during propagation. Even when an effective fungicide is used more disease is spread this way than is controlled. Do a sprench after sticking cuttings to avoid spreading disease around and get the best preventive control.

Ann R. Chase is a well-known plant pathologist and expert on diseases of ornamental plants throughout the United States. She is a widely published author, teacher and speaker.

Photos courtesy of Ann Chase