Introduction
Managing crop pests and diseases is a critical challenge for farmers. Rising demands for high-quality produce and population growth have increased the need for effective management practices. While chemical control has long been a favored method due to its rapid action and affordability, overuse has caused severe health and environmental issues. These include ecological imbalances, pesticide residues, and the emergence of resistant pathogens. Fusarium oxysporum, a fungal pathogen targeting Solanaceae crops, exemplifies this resistance and poses significant control challenges. Consequently, biological control methods are gaining traction as sustainable alternatives.
Understanding Soilborne Pathogens Physiology
Soilborne pathogens, microorganisms thriving in soil, infect plant roots under favorable conditions, leading to impaired water movement, wilting, and substantial crop losses. Among these, Fusarium oxysporum and Verticillium dahliae are known for their long survival periods, their ability to infect diverse crops, and the severity of the symptoms they cause. These fungi can survive in the soil for extended periods without a host (many years), making their control particularly challenging.
Laboratory of Plant Pathology, Aristotle University of Thessaloniki (AUTH), 2019 - Dimitra Kogioumtzidou
Soilborne phytopathogenic fungi survive in the saprophytic phase of their life cycle within the soil for most of their lifespan, either existing freely or in plant residues. While they infect the below-ground plant parts (root system), the symptoms of the diseases they cause appear mainly in the above-ground parts. High relative humidity, low temperatures, poorly drained soils, and high nutrient concentrations due to excessive fertilization are favorable conditions for infections. Diseases caused by soilborne pathogenic fungi include collar rots, root rots, damping-off, and vascular wilts, affecting both perennial and annual crops. Typically, affected plants appear in patches in the field, often exhibiting symptoms of partial or total wilting.
Fusarium oxysporum f.sp. radicis-lycopersici (Forl)
The fungus Fusarium oxysporum f.sp. radicis-lycopersici (Forl) is one of the most important plant pathogens worldwide, responsible for causing vascular wilts and dry rots, primarily in plants of the Solanaceae family, leading to devastating economic losses. Vascular wilts are diseases affecting the plant's vascular system, specifically the xylem, caused by infections from fungi in the genera Fusarium spp. and Verticillium spp..
The pathogen infects a wide range of hosts and acts as a highly specialized parasite, targeting a specific host or host group in a particular form, known as forma specialis. Each forma specialis, or pathotype, may consist of strains adapted to specific host varieties, as seen with Fusarium oxysporum f.sp. radicis-lycopersici. Over 70 formae speciales capable of causing vascular wilt in numerous annual crops have been described. In tomatoes specifically, two formae speciales have been identified:
- Fusarium oxysporum f.sp. lycopersici (Fol) and
- Fusarium oxysporum f.sp. radicis-lycopersici (Forl),
They differ in terms of the symptoms they cause in the host plant, their movement rate through the plant's vascular system, and the conditions required for their growth and spread.
Fusarium infection in tomato plants
Fusarium oxysporum f.sp. radicis-lycopersici (Forl) infects young tomato plants in the seedbed and shortly after transplanting, but such infections rarely lead to complete wilting of seedlings. In contrast, in "mature" greenhouse crops, infections tend to appear just before full ripening and harvest, typically under conditions of sunlight and high temperatures.
The initial symptoms of vascular wilt caused by Fusarium oxysporum f.sp. radicis-lycopersici (Forl) resemble typical soilborne or generalized disease symptomatology. These include stunted plant growth, signs of early wilting and epinasty, mild chlorosis on stems and leaves, drying of apical parts, and slowed growth rates. Generally, vascular wilt, like other soilborne diseases, can appear in two forms: as partial wilting, where the plant gradually dries while retaining productivity for some time, or as a sudden collapse due to vascular blockage.
Distinctive pathological symptoms are observed in the collar and root area, where brown rot of the bark and peeling of the stem's outer layers occur. Dark discoloration of the vascular tissue extends upward from the root along the stem to about 25cm (9.8 inches), but not beyond, while leaves exhibit diffuse yellowing with retained green color. The continuous brown discoloration of the vascular parenchyma becomes visible through cross-sectional cuts with specialized tools on infected plant parts. Roots develop numerous dark necrotic lesions, gradually leading to general root system discoloration and complete root rot.
In advanced stages, brown ulcers appear at the plant's collar, where red fungal fruiting bodies may form under favorable temperature and humidity conditions, in the form of cottony mycelium. In cases of advanced rot, secondary lateral roots develop near the collar, the primary seminal root may be lost, upper stem sections thin, and infected fruits soften and gradually lose their natural vibrant color.
Pathogen Entry and Spread
The pathogen enters through root wounds, natural openings, or nematode-feeding sites. It spreads upward through the cortical tissue, reaching xylem vessels. Regarding environmental conditions, temperatures between 18-26°C (64.4-79 °F), with an optimal range of 20-24°C (68-75 °F), are considered favorable for disease development. Additionally, the spread of the pathogen and the severity of infections are accelerated in sandy, acidic soils with a pH below 7, especially in those treated with steam or chemical disinfectants, poorly drained, and with high concentrations of salts and ammonium nitrogen.
Effective Management of Soilborne Diseases
To address soilborne diseases, integrated pest management (IPM) combines legislative, cultural, chemical, biological, and varietal strategies to minimize environmental and health impacts.
- Legislative Measures:
These include implementing legislative actions at both national and international levels, in accordance with the International Plant Protection Convention of 1951. They establish phytosanitary controls to prevent the introduction of new unwanted plant pests into an area and timely communicate to producers about the necessary actions to take to limit the spread of an already established plant pathogen. A significant measure within this category is the use of certified propagating material. - Cultural Practices:
These are the set of actions applied by the grower, aiming to create conditions favorable for the growth of cultivated plants while simultaneously unfavorable for the development of plant pathogens.
Growers can implement:
- Crop rotation with resistant varieties.
- Pruning and removal of infected residues.
- Proper soil management to regulate moisture, acidity, and salinity.
Preventive measures include disinfection of tools and avoidance of root damage.
- Chemical Methods:
The use of chemical agents is considered necessary, as they have been found to control pests of all kinds, except for viruses, and in some cases, not using them makes production impossible or unprofitable. However, in the case of soilborne pathogens, their effectiveness is particularly weak due to the ability of these pathogens to survive in the soil for long periods of time. Furthermore, the ban on certain benzimidazole and fumigant fungicides makes chemical control of Fusarium oxysporum f.sp. radicis-lycopersici is impossible, which is why implementing preventive control methods is deemed essential. - Resistant Varieties:
In addition to ensuring healthy propagation material, using resistant rootstocks and varieties is also considered legitimate. This method addresses plant pests by cultivating varieties that do not incur economic damage even after infection; a trait passed on to offspring. Although it seems a cost-effective method, it has proven to be of uncertain outcome due to its dependence on the available pathogen load in the field and the prevailing environmental conditions. Specifically, no resistant varieties have been reported for Fusarium oxysporum f.sp. radicis-lycopersici (Forl) to date. - Biological Control:
Significant efforts in plant protection for the development of biological control have been ongoing since the 1960s. While progress has been made, the success rates of effectively controlling plant pathogens using exclusively biological agents remain low. Specifically, the biological control of Fusarium oxysporum f.sp. radicis-lycopersici is still in the experimental stage. However, the method is considered promising, as several biological agents have shown inhibitory activity against this soilborne fungus, particularly with the use of rhizobacteria.
Rhizobacteria are soil bacteria that develop near plant tissues (roots, root nodules, root hairs) and stimulate plant growth. They are known as PGPR (Plant Growth-Promoting Rhizobacteria) and can exhibit significant antifungal activity in addition to promoting plant growth. Examples of biocontrol agents tested against Fusarium are presented in the table below, along with relevant bibliographic references.
Conclusion
Effective management of Fusarium oxysporum f.sp. radicis-lycopersici requires an integrated approach, combining legislative, cultural, and emerging biological strategies. As research progresses, biological control methods hold significant potential for sustainable pathogen management, reducing reliance on chemical inputs while protecting crops and ecosystems.
This article has been created with information from the author's thesis: Organic extracts from liquid cultures of the rhizobacteria Pseudomonas chlororaphis (ToZa7), were studied for their ability to inhibit conidia germination of the plant pathogenic fungus Fusarium oxysporum f.sp. radicis-lycopersici (Forl).
Biocontrol Agents References
Pseudomonas fluorescens WC5365 Dekkers et al., 2000
Pseudomonas chlororaphis RCL1391 Chin-A-Woeng et al., 1998
Pseudomonas chlororaphis PCL1751 Kamilova et al., 2005
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Further reading
