The term Fusarium wilt may be unique to most of us, but it is a very familiar term to farmers and agriculturalists. Fusarium is a genus of fungi; some species are plant pathogens and some are opportunistic infectious agents of humans and other animals. There are actually over fifty different species, depending on the host plant infected. It is referred to as a soil pathogen because it is usually carried and passed by the soil.
Fusarium is found all around the world, infecting plants when conditions are opportune, such as stagnant water and over-moist soil with bad drainage. The actual pathogen is Fusarium oxysporum, which is further differentiated into different forma specialis (sub-species). The symptoms on the infected plants are wilting, chlorosis (yellowing or blanching), necrosis (tissue death), premature leaf drop, browning of the vascular (stem) system, stunting and damping-off (decay and death due to excessive moisture). The most important, indicative and noticeable symptom is the actual vascular wilting. Older leaves start to droop, then the plant stops growing, starts yellowing, and eventually dies. It usually occurs between the blossom and fruit-forming stages.
Fusarium oxysporum can attack sweet potatoes, tomatoes, bananas, pears, pigeon peas, muskmelons, cantaloupe, tobacco, legumes, basil, beans, carnation, chrysanthemums, watermelon, peppers, cucurbits (a type of gourd) and cereals. It infects a healthy plant by mycelia or spores penetrating roots. After the plant dies the fungus invades all tissues, sporulates (forms spores), and continues to infect neighboring plants. F. oxysporum spreads fastest through soils that have high moisture and bad drainage. Fusarium can be in farming soil, or, indoors, in carpet and mattress dust, damp walls, humidifier pans and other locations of stagnant water. Some species’ spores are inhalant, which can affect animals, livestock and humans.
The concern for agriculturalists is that if one plant succumbs, the soil, as the vector, will eventually allow it to spread, ruining entire crops. And this condition cannot be resolved by crop rotation. Also, as with many pathogens, the fungus can become more virulent, thereby attacking plants considered resistant. Fusarium can live in the soil for long periods of time as well as through infected dead plant tissue. Infected farms need to do a total “clean up” of the crop area. This disease control would begin by removing all infected plants, improving soil conditions, using resistant plants where available, and applying soil and systemic fungicides on the infected soil. Other methods which should be utilized include flood fallowing and using clean seeds rather than seeds from previous crops. A biologic control has not been determined, since closed environments such as greenhouses do not simulate the open-field environments in which it is commonly found. For melons, there has been success in grafting a susceptible variety onto resistant root stock.
Not all news is bad:
“There is growing interest in using Fusarium wilt as a form of biological control. Certain pathogenic strains of F. oxysporum could be released to infect and control invasive weed species. This type of control (called a mycoherbicide) would be more targeted than herbicide applications, without the associated problems of chemical use. In addition. F. oxysporum may compete with other soil fungi that act as pathogens of important crops. Introducing specific strains of F. oxysporum that are not pathogenic (or non-infectious mutants of pathogens) to nearby crops could take nutrients from other potential disease-causing fungi” (Snyder, W.C. and Hansen, H.N. 1940. The species concept in Fusarium. Amer. J. Bot. 27:64-67.)
Not only is it economically ruinous to lose entire crops of fruit to this ubiquitous fungus, but there is a chance of infecting animals and human beings, through eating the plants and fruits or by inhaling the spores. Exposure to Fusarium can also occur through the skin at damaged sites such as wounds and burns.
In human beings, the most common problem is allergy to molds and fungi. If a person is strong immunologically, the symptoms are minor and usually local. There may be other symptoms, but the condition is very treatable and survival rate of a systemic infection is 67%.
However, if a person is immunosuppressed as a result of transplants or excessive corticosteroid use, the consequences can be serious. If the patient has prolonged and profound neutropenia (a decrease in white blood cells called neutrophils) or a T-cell deficiency, or is under HSCT (Hematopoietic stem cell transplantation ) treatment, the symptoms can include cutaneous lesions, sinusitis, endophtalmitis (an inflammation of the inner eye), and/or pneumonia. Unfortunately, the prognosis for survival in compromised patients is 0% to 4% in patients with more than one compromising condition or persistent neutropenia. With only corticosteroid therapy, the survival rate climbs to 30%.
Treatments for infections vary according to the condition of the patient and the symptoms presented; there is no standard approach and a limited number of clinical trials available. The best treatment is prevention, and when a patient is at risk, extensive protective measures are taken.
In conclusion, Fusarium wilt is known but still omnipresent. It can cost farmers entire crops, inflate fruit prices, and even threaten wildlife and people. Therefore it is not to be dismissed and efforts need to be made worldwide to prevent conditions which can spread the fungus.