Use diligence to control Fusarium outbreaks

Fusarium can cause very different symptoms on different plants under different environmental conditions. On some tropical plants, the pathogen can cause leaf spots, while on others (like cyclamen) vascular wilt is the most common symptom.

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In the Chase Horticultural Research diagnostic lab, we see Fusarium spp. causing cutting rot, crown rot and stem rot more than anything else on woody plants, perennials and bedding plants. Watch for Fusarium on basil, phormium, chrysanthemum, dianthus, mandevilla, lisianthus, cyclamen, dracaena, dieffenbachia and Christmas cactus (Schlumbergera truncata).

New Fusarium diseases

One of the most intractable diseases we have encountered in the lab is dieback of mandevilla and phormium caused by Fusarium spp. On both plants, Fusarium spores are spread during propagation. In the case of phormium, the disease shows up as a rot of the stems and petioles at the stem tips. Unfortunately, sometimes the symptoms are minor and phormium plants with petiole rot are used as divisions for the next crop.

In contrast, Fusarium kills mandevilla, especially the Sun Parasol series. This disease is also spread by propagation. We have seen flats of well-rooted mandevilla cuttings with very few signs of problems slowly succumb to Fusarium dieback. The disease appears to be greatly affected by shipping stress and wounds that occur during that process.

Fusarium spread

The first step toward solving a Fusarium problem is to understand its source as well as the cause of the disease. Over this past year I have learned that Fusarium infections can escape detection until plants are shipped. Diseased plants are sometimes obvious in propagation while at other times it takes weeks for the disease to develop. This can make determining when and where the disease started very difficult.

Fusarium is a soil-borne fungus that usually produces lots of spores that are somewhat sticky. They are easily spread by propagation tools and water splashing.

One of the most effective ways to spread the pathogen evenly through a crop is to dip unrooted cuttings into a fungicide or plant growth regulator bath. In these situations the spores that might only contaminate less than 1 percent of the cuttings are 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 selected is too low or the fungicide solution is kept too long. Once a surface is contaminated with Fusarium it is very hard to eradicate since this fungus is a good saprophyte. This means it does not have to live on a plant and not all Fusarium species are even plant pathogens.

Killing Fusarium spores

There are quite a few products used to disinfest greenhouse surfaces including benches, floors and trays. These include bleach (sodium hypochlorite), peroxides (i.e., ZeroTol), quaternary ammoniums (i.e., GreenShield, Physan and KleenGrow) and chlorine dioxide. In 2005, we tested some of these using chlorinated cleaner PAC (also known as Horti-Klor from Pace Chemicals) before applying KleenGrow, GreenShield or ZeroTol. We tested these products on 4-mil polyethylene film, wood and concrete block with Fusarium oxysporum fsp. cyclaminis (the cause of Fusarium wilt on cyclamen). Surfaces were dirty with obvious algae growth when the study was started.

PAC was applied to each surface as a spray (5 ounces per gallon) and rinsed after five minutes. This was followed by treatment with one of the disinfestants: KleenGrow (1 ounce per gallon), GreenShield (1 tablespoon per gallon) 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 was the most difficult.

PAC alone was somewhat effective. KleenGrow was very effective on all three surfaces when 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 the least effective in this test with mixed results on the different surfaces.

In a follow-up test, we evaluated the effect of treatment timing on pathogen longevity on wood. Wood was inoculated with F. oxysporum one hour before cleaning with Strip-It, an acid-based heavy-duty cleaner. This was followed by an hour or a day later with KleenGrow. Recovery of Fusarium was attempted one day after this treatment and again one week later.

Delaying the disinfecting treatment by 24 hours did not result in increased levels of Fusarium. When the treatments were one hour apart no recovery occurred. After one week, there was more Fusarium on the water-treated controls than was found after a single day, showing that it actually did multiply on the dirty wood surface.

The ability of Strip-It and KleenGrow to kill Fusarium was about the same after one week as it was after one day. This indicates that there was little re-growth on the treated surface during the interim.

Fusarium control on specific crops

Christmas cactus. Fusarium rot of cacti and succulents is usually caused by Fusarium oxysporum and includes dieback and root and stem rot. In 2006, we performed a series of trials with some new fungicides for control of cladophyll (stem segments) rot on Christmas cactus (Schlumbergera truncata). This disease is very difficult to stop on Christmas cactus. The best products in the three trials were Insignia and Heritage. None of the other fungicides we tested gave enough control to make their use worthwhile.

Phormium. Two trials on Fusarium crown rot on phormium were run using phormium divisions, which showed active infection with Fusarium, planted in 1-gallon containers. The plants were drenched (or sprenched) six times on a 14-day interval. The number of rotted leaves per plant was recorded every few weeks. Table 1 shows the summary of the two trials. Not all products were tested in both trials.

The only products that appeared to provide some control were Medallion, Pageant (a new combination product from BASF that should be registered for ornamentals this year) and Heritage. Trying to stop an active Fusarium infection has proven even more difficult than preventing one on many crops.

Cyclamen. We also conducted a couple of trials on Fusarium wilt on cyclamen. The pathogen is F. oxysporum fsp. cyclaminis and causes yellowing of lower leaves, wilting, root rot, browning or purpling of the vascular tissue in the corms and sometimes corm rot. Fungicides drenches were applied five or six times on a 14-day interval. In the first trial, some phosphonates (Aliette, Alude, Fosphite, pHortress and Vital) were evaluated for preventive control of Fusarium wilt. Last year, we obtained an indicator that control was possible, but our trial this year showed no control. Two of the products (pHortress and Fosphite) were worse than the inoculated control alone.

In the second trial, two new products from BASF (Trinity and Pageant) showed very good results although the Trinity rates did result in phytotoxicity after four drenches. Trinity is a sterol inhibitor currently labeled for turf only.

Medallion performed rather poorly in these cyclamen trials despite the fact that previous studies over the past 10 years have shown it to be one of the best products for this disease.

Fungicide efficacy control of Fusarium stem rot

Treatment	Amount applied	Christmas cactus	Phormium	Cyclamen
Actino Iron	5 pounds per cubic yard of medium

None (highest-quality plants in trial)

Actinovate	6 oz./100 gal.	Slight	None

Aliette	16 oz./100 gal.

	None
Alude	64 oz./100 gal.

	None
Chipco 26019	16 oz./100 gal.	Some

Cleary 3336	16 oz./100 gal.	None

Compass O	4 oz./100 gal.	Some	None

Fosphite	32 oz./100 gal.

	Some	Worse than water
Heritage	4 oz./100 gal.	Good	Good	Good (previous trials were very good)
Insignia	8 or 10 oz./100 gal.	Very good	Some	Some
Medallion	4 oz./100 gal.	Some	Very good	None (previous trials were very good)
Pageant	8 and 12.5 oz./100 gal.

	Very good	Very good
pHortress	64 oz./100 gal.

	Worse than water
Rhapsody	128 and 256 oz./100 gal.

None

Terraclor	8 oz./100 gal.	None

Terraguard	4 oz./100 gal.	Slight

	Previous trials were very good
Trinity	6 and 12 oz./100 gal.	Some	Some	Very good
Vital	64 oz./100 gal.

None

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- Ann Chase