By Dr. Thomas T. Yamashita
Fusarium solani is a pathogenic fungus with a broad host range, attacking the root and crown regions of plants such as tomatoes, beans, lettuce, sweet potato, pepper, asparagus, alfalfa, cotton, dieffenbachia, caladium, and many others. It has also been observed to attack seeds and seedlings in the early stages of development, being responsible for damping off diseases of various crop plants. It also colonizes the tubers of sweet potatoes and russet potatoes, causing a storage disease known as dry rot.
Thanks to its broad range of host species, this soil-borne fungus has established an extensive geographical distribution. However, F. solani is a relatively weak pathogen, and in years with comparatively normal growing conditions, the pathogen may be inconspicuous or seen only occasionally.
But environmental or physical conditions which weaken plants also set the stage for infections by F. solani.
Some years ago, wet soil conditions late into planting season resulted in crops being planted late. Soon after the seedlings emerged, temperatures shifted from cool to over 100 degrees Fahrenheit. Many plants literally shut down photosynthesis and were physiologically stressed as respiration (food utilization) was escalated. Due to the sudden and intense heat, many farmers could not properly coordinate their irrigation schemes. Many stressed their crops in their attempts to compensate by using the 2- to 3-week intervals typical of normal weather, instead of the 7- to 10-day interval demanded by the high temperatures. Others overcompensated, keeping their soils excessively wet, compounding the problem even more by inducing oxygen stress. These are exactly the conditions which eventually favor infections by F. solani.
A complication that growers often run into is that most conventional fungicides are directed at water mold fungi, such as Pythium or Phytophthora. Most of these water mold fungicides are highly specific and offer little to no protection against other groups of fungi, including F. solani. Some growers will respond by including a fungicide called PCNB (pentachloronitrobenzene), which is directed at another fungal soil pathogen, Rhizoctonia solani. PCNB, however, has little activity against F. Solani.
All these factors aligned after the rains of 1995 and 1996, when California came off 7 years of drought. During this period, we saw a rapid proliferation of many soil-borne pathogens, including F. solani. The stage was set for outbreaks of F. solani, due to:
- The build-up of high populations of F. Solani in the soil
- Predisposing environmental stresses
- Selective minimization of competition by targeted fungicides
We saw the consequences of these circumstances firsthand.
Growers attempting to mitigate water mold infestations often find themselves blindsided by Fusarium solani.
In 1996, we investigated several pepper fields which had been treated with water mold fungicides. We found the fungicide to be highly effective, as the plants were virtually free of water mold infections. Yet, the plants were wilting in the middle of summer, with every manifestation of a typical water mold infection. Closer examination of the roots and crown (region of the stem near the soil line), revealed heavy infections of both F. solani and Rhizoctonia solani. Soil populations of both pathogens were high, while levels of beneficial microbes were very low.
On another occasion, we examined a test plot for processing tomatoes, beautifully and painstakingly designed to test the water mold-protecting properties of a superior fungicide. The investigator, a person with exceptional acumen for research, had mapped out and treated, to the row, the low-lying area of the tomato field. The rationale for this was the knowledge that water mold pathogen pressures would be highest in the wetter areas of the field.
A week before harvest, seemingly overnight, the drier and untreated side of the field literally burned up, succumbing to disease. The treated plot, however, remained green and thriving. The test plot had all the makings of a textbook, picture-perfect result. But the investigator realized that unexpected factors were at work, as there were also healthy plants in the untreated control area. We collected samples and found that in all cases the affected tomato plants hosted heavy infections of F. solani and R. solani. As with the previous case, soil populations of the two pathogens were high, and beneficial microbes were practically nonexistent.
The tomato field was on ration with beans, a common host of the two fungi. Heavy populations had built up in the soil, and when the heat wave struck, the tomato plants suffered severe water stress. When the farmer started to back off on his irrigation to prepare the ground for harvest, the pathogen populations practically exploded and overcame the plants. The part of the tomato block selected for the test plot was in a naturally moist area of the field, which delayed the total collapse of these plants. Additionally, the relatively moist conditions of the soil in the test plot supported a higher population of beneficial soil microbes and a proportionally lower population of the pathogens.
Combatting Fusarium solani requires knowing that such pathogens prosper in sterile soils lacking beneficial microbes and take advantage of pre-weakened plants.
The first step in combatting soil-borne pathogens such as F. solani involves reducing their population levels.
Earlier in their evolution, many such pathogens were free-living microbes that lived on plant debris. However, competition with other microorganisms leads these pathogens to develop more specialized types of enzymes that allow them to enter a living plant, minimizing their exposure to a competitive environment. As these pathogens become ever more specialized, they lose their ability to survive on a larger array of dead tissue. In short, they lose their tolerance. Deprived of a host, they can survive in soil, but not thrive.
Hence, increasing the population of beneficial microbes increases competition, markedly reducing pathogen populations. This is why we suggest that prior to postharvest cultivation, growers should spray plant tissues with specially selected strains of microbes known to be antagonistic to problematic pathogens. This is best done in conjunction with a liquid food base which the beneficial microbes can feed on. However, this can be omitted when using microbes that can directly feed on the pathogenic organisms in question.
At or prior to planting, one can gain additional protection against F. solani by banding a broad-spectrum fungicide in conjunction with a bacterial antagonist (e.g. Botran + Pseudomonas fluorescens). The fungicide curbs growth of most fungi, but does not affect the introduced beneficial bacterial, providing additional protection for bacterial proliferation. The material can be sprayed and incorporated via rototilling, or band-injected beneath the seed or seedlings. Most of these beneficial bacteria thrive in a pH range of 7.0 to 7.5. You may need to adjust with lime or fertilizer to get closer to this range.
It’s also important to remember that F. solani is an opportunistic organism, waiting for a weakening event to make host species vulnerable.
Well-balanced plant nutrition and proper irrigation are critical in combatting this disease. It is imperative to pay particular attention to calcium (Ca), potassium (K), phosphorus (P), and boron (B) levels within the soil. These levels should be at or above:
- Ca: 600 ppm
- K: 200 ppm
- P: 35 ppm (note: this must be readily available P, often referred to as P1)
- B: 0.6 ppm
Corresponding leaf tissue levels should be at or above:
- Ca: 2%
- K: 2.5%
- P: 0.25% (again, readily available P1)
- B: 40 ppm
Fruit-bearing or field crops should also be examined for mineral levels in the fruit flesh. If the fruit is a tomato, for example, begin monitoring fruit flesh levels at the small golf-ball stage. Wash the fruit skin to free it of surface contaminants, pull off the stem, and dice the tissue into small cubes. Place this into a sample bag and submit it to your agricultural testing lab of choice with a request for a dry tissue analysis for N, P, K, Ca and B.
Going forward, establish a desirable N:Ca and Ca:B ratio (we’ve spoken at length previously about the role of calcium and boron in plant development). Monitor the other minerals for absolute values. The importance of this practice resides in knowing that while the leaf tissues may show high levels of the monitored minerals, the fruit will be somewhat deficient. Nitrogen will typically be 1/4 to 1/3 of leaf levels, phosphorus will be 1/3 of leaf levels, potassium will be about 85% of leaf levels, calcium will be 1/50 to 1/40 of leaf levels, and boron will be about 80% of leaf levels.
Take fruit samples biweekly and establish a graph of absolute values, as well as N:Ca and Ca:B ratios. Ideal N:Ca ratios will vary slightly for different crops but, in most fruit, should run 15:1 or less. Ideal Ca:B ratios will also vary for different crops but, in most fruit, should be close to 25:1. The N:Ca ratios in the leaf tissues ideally should be close to 1:1. Ca:B ratios in the leaf tissues ideally should be about between 400:1 and 500:1. However, calcium is poorly harvested during rapid growth (such as occurs during heat waves), meaning it is important to apply calcium plus a minute quantity of boron ahead of and during heat waves, as well as behind mild to heavy applications of nitrogen. This calcium must be applied through foliar sprays in addition to soil applications. When calcium is sprayed, it would prudent to include other needed minerals as indicated by tissue analysis.
Without proper irrigation, the crop can be totally lost or dramatically reduced. Learn the characteristics and boundaries of your soils and avoid stress. Remember that water stress can take the form of excessive irrigation as well as under-irrigation. For example, during heat waves, excessive irrigation and the creation of standing water conditions will wreak havoc on crops, in some cases killing the plants. Short, frequent pulses are better than long, heavy doses spaced farther apart in time. During heat waves, sandy soils will require 3- to 5-day intervals between irrigation pulses. In the same conditions, heavier soils will require 7- to 10-day intervals.
Adopting this approach, as summarized below, will help to significantly curb Fusarium solani and resulting Fusarium crown and root rot.
- Before cultivating plants, spray them with antagonistic organisms plus liquid substrate, and encourage the buildup of beneficial, antagonistic microbes in planted soils.
- Establish sound soil levels of Ca, K, P and B.
- In the planting bed, apply a broad-spectrum fungicide and establish desirable levels of antagonistic bacteria.
- Maintain sound irrigation schemes, avoiding over- or under-irrigation at all times.
- Supplement with foliar calcium and minute quantities of boron, especially ahead of and during heat waves.
Note: If population levels of F. solani are especially high, you may have to define the boundaries of heavy soil populations and spot fumigate to bring down populations. Following dissipation of the fumigant, plant a green manure and incorporate beneficial antagonists with liquid substrate at cultivation or reestablish beneficial antagonistic microbe populations in bed treatments.