Fighting Fusarium on Cut Flowers

This project was supported by the American Floral Endowment in collaboration with Dr.  Robert McGovern (University of Florida) and Dr. David Geiser (Pennsylvania State University)

Fusarium diseases can be especially troublesome for cut flower growers.  Crops like chrysanthemum, China aster, dianthus (carnation and sweet William), gerbera, gladiolus, iris, lily, lisianthus, and narcissus are all susceptible to fusarium diseases.  Most diseases are caused by one of three different species of the fungus Fusarium Fusarium oxysporum and F. solani are actually complexes of many specialized strains, some of which are called formae speciales (f. sp.) or special forms.  These species are associated with wilts and rots of corms, bulbs, stems, and roots.  Another species, called F. avenaceum, appears to be less specialized, but can still cause destructive stem and crown rots on chrysanthemum and lisianthus.  
Symptoms frequently associated with fusarium diseases are wilt, yellowing, and eventual death of the lower leaves. Highly susceptible plants will collapse and die. Occasionally, plants will develop these symptoms only on one side of the plant. Most of the wilt diseases also cause a reddish-brown discoloration in water-conducting tissues in the roots and stems. Although this symptom can be diagnostic for Fusarium, growers should get laboratory confirmation before initiating control treatments.

Keep it Clean

The best strategy for managing fusarium is to use resistant cultivars. Unfortunately, very few flower cultivars have been identified that have good resistance to fusarium and limited data are available.  Four China aster cultivars from Stokes Seed Company—Bouquet Puff Mix 2080, Stokes Aster Sandy Mix, Finest Mixed 684, and Astoria Mix Aster 2087—were found to have moderate levels of resistance to fusarium wilt (Table 1). Another study conducted in Europe found that the Asiatic lily cultivars ‘Orlito’ and ‘Connecticut King’ were highly resistant to fusarium bulb rot. More of these kinds of studies are needed to better understand the genetics of Fusarium resistance in different flower crops. This could lead to breeding resistant genes into the more popular cultivars.
When resistant cultivars are not available, growers must practice strict sanitation.  Many fusarium diseases infect their hosts at the beginning of the crop cycle, even though symptoms might not appear until flowering. Once the pathogen is present in a greenhouse or field, it can move short distances in a number of ways. Spores can be spread by fungus gnats, in splashing rain or irrigation water, and on soil, pots, and trays. We found that most disinfectants were effective in eliminating the pathogen from Styrofoam trays, but bleach, Lysol, and peroxyacetic acid reduced the fungus to undetectable levels.
Fusarium pathogens can also be dispersed for long distances on seeds, plugs, and/or transplants.  Eliminating this primary inoculum is the key to managing fusarium diseases.  Therefore, the first line of defense in management of fusarium diseases should focus on sanitation because once the disease has become established in an operation, it becomes increasingly difficult to manage. Our observations found that seeds of China asters sold commercially were frequently infested with the fusarium wilt pathogen (F. oxysporum f. sp. callistephi). Out of 25 seed packages of China aster, we found 7 (28%) had the fungus on the seed coats. Disinfesting seeds in 1% household bleach for 30 minutes eliminated the fungus in our trials. Growers should always ask seed companies if disinfested or treated seeds are available. Many times, corms and bulbs can also have fusarium infections that can lead to major losses.
Although disinfestation procedures (e.g., hot water, fungicide soaks) are available for some corms and bulbs, growers should exercise caution and pay close attention to instructions when applying these treatments.  Since it is difficult to tell whether plugs and transplants are infected when no symptoms are present, growers must be vigilant in keeping track of purchased plant material. If symptoms appear, symptomatic plants should be removed. If fungicides are used, they should be applied to the remaining crop according to the label. Symptomatic plants should be discarded—do not compost them since many Fusarium species can produce long-term resting spores called chlamydospores that can survive the composting process.

Watch Your Rotation
Once Fusarium has become established in the soil, a cut flower grower can be forced to abandon production of that flower crop.  There are also claims that the pathogens can persist in soil for up to 10 years, even in the absence of a susceptible flower crop.  However, since most fusarium diseases are highly specific to their own host, this means that fields with a history of fusarium wilt on crop “A” could likely be planted with crop “B”, provided crop “B” is not closely related to crop “A.”  For example, fields where fusarium wilt of sweet  William had occurred could safely be planted to gladiolus, chrysanthemum, or China aster, and they would not become infected.
Growers also have some cultural options to manage fusarium diseases.  Nutrition can play an important role in reducing the severity of fusarium diseases. For example, fusarium wilts and root rots are less severe when the nitrogen (N) is applied as nitrate (NO3) and more severe when applied as ammonium (NH4).  These N-forms are metabolized differently by plants and can have opposite effects on the severity of fusarium diseases. Much of the effect governing fusarium suppression with nitrate relates to the soil pH in the root zone. Nitrate-N tends to raise soil pH, whereas ammonium-N tends to lower soil pH. As a consequence, liming can also reduce fusarium diseases. These effects occur through chemical reactions in the soil as well as root-mediated exchanges in the root zone. Growers should try to keep soil pH around 6.5-7.0.  Fusarium diseases are also more severe when plants are under stress. Soils kept too wet or too dry can favor the disease by stressing the plant. Similarly, temperatures too warm or too cold can predispose the plant to infection.
Start with the Soil

Efforts to increase soil health in the field can reduce the severity of fusarium wilt and other soilborne diseases. Winter cover crops, such as clover, rye, and/or vetch, have been found to reduce the severity of fusarium wilts and can be useful in providing organic matter for improved plant growth.  Research at The Connecticut Agricultural Experiment Station has demonstrated that maintaining a large earthworm population in the soil can also reduce fusarium diseases. Growers can increase the number of earthworms in their soils by practicing no-till or minimum tillage, minimizing chemical inputs, and applying sufficient lime.   
There are a few fungicides that can protect plants from fusarium diseases. Products like Heritage® (azoxystrobin), Medallion® (fludioxonil), and Terraguard® (triflumizole) have been shown to provide some protection in the greenhouse. However, if the plant is already infected, these products may just delay the onset of symptoms.  Products containing thiophanate methyl e.g., Domain®, Cleary’s 3336®, Banrot® do not provide as much protection.  Biological fungicides, such as Actinovate®, Companion®, MycoStop®, or RootShield®, cannot stand alone, but have been shown to suppress fusarium when combined with chemical fungicides.  If they are used, they should be applied with or following a fungicide drench.
In summary, cut flower growers have a few options for dealing with a fusarium disease.  Sanitation and resistant cultivars are the two most successful strategies, but when these options fail, early scouting and quick action is necessary to minimize spread.  It is reasonable to assume that more fusarium diseases will appear in the future as demand for new cultivars encourages more seed distribution. This means that other popular cut flowers crops that have no serious fusarium disease problems today might develop one in the future.  For example, fusarium wilt of coreopsis was found and reported for the first time in 2007. As for all pest problems, paying close attention to your flowers and getting professional diagnostic help when needed is the key for managing a healthy crop.

Wade Elmer

Plant Pathologist

Wade Elmer is Plant Pathologist with The Connecticut Agricultural Experiment Station. Contact him at [email protected]