Fungal Diseases - Fusarium Wilt, Wilt of cotton

 

Diseases are a major limiting factor for crop production. Diseases can be caused by infectious microorganisms that include fungi, bacteria, viruses and nematodes. These diseases are contagious and can spread from plant to plant in a field, often very rapidly when environmental conditions are favorable.  

1) Fusarium Wilt

Organism - Fusarium oxysporum f.sp. lycopersici 

Wilt diseases are caused by pathogens that invade the vascular system (xylem tissue) and disrupt water flow through the plant. Fusarium wilt is the major wilt disease of tomato.  This is a warm-weather disease caused by the fungus Fusarium oxysporum.

Symptom

• The first symptom of the disease is drooping and wilting of lower leaves with a loss of green colour (chlorosis)
• Affected leaflets wilt and die. The younger leaves may die in succession and symptoms continue in subsequent leaves.
• Often leaves on only one side of the stem turn golden yellow at first.
• The disease progresses up the stem. At later stage, browning of vascular system occurs.
•  The stem of wilted plants shows no soft decay, but when cut lengthwise, the lower stem will have a dark brown discoloration of the water-conducting vessels.
• Soon the petiole and the leaves droop and wilt until all of the foliage is killed and the plant becomes stunted and dies.

Transmission

• The fungus is soil-borne and passes upward from the roots into the water-conducting system of the stem. Blocking of the water-conducting vessels is the main reason for wilting.
• The fungus survives and persists indefinitely in field soil.
• Invasion occurs through wounds in roots growing through infested soil.
• Long-distance spread is through seed and transplants.
• Soils become infested by planting infected transplants and from movement of infested soil by wind and water erosion or on farm implements.

Prevention & Treatment:

• The affected plants should be removed and destroyed.
• For control, grow plants in pathogen-free soil
• Use disease-free transplants
• Grow only cultivars with resistance to Fusarium wilt. Growing tomato varieties resistant to Fusarium wilt is the most effective means of control.
• Crop rotation with a non-host crop such as cereals is also an effective control method
• Spot drench with Carbendazim (0.1%)

 

2) Wilt of cotton

Organism - Fusarium oxysporum f.sp. vasinfectum & Verticillium dahlia

 

Vascular wilts, such as Fusarium wilt and Verticillium wilt, are capable of significantly reducing cotton yields, and negatively impacting fiber quality. Properly diagnosing these diseases is critical in developing a management system. While subtle differences can be observed in the field, laboratory examinations are often required to differentiate the two diseases. Variety selection is the most effective factor in reducing wilt associated losses.

Fusarium wilt

Causal agent: Fusarium oxysporum f. sp. vasinfectum (Fov)

Disease development depends on host susceptibility and pathogen - infection by Fov is more if root-knot nematode (Meloidogyne incognita)  infection is present.  Disease occurs in warmer temperatures and sandy soils.

Fusarium can cause severe symptoms in susceptible cotton varieties. These include a general wilt and yellowing and necrosis of lower leaf margins. The vascular system of infected plants is discolored brown in affected portions of the tissue. This is most apparent in the lower stem and upper taproot. The discoloration starts in the taproot, spreads into the stem, and is generally continuous in contrast to the speckling nature of the discoloration in plants affected by Verticillium wilt.

Symptoms

occur throughout the growing season

• Initial symptoms consist of chlorosis and wilting on the margin of the leaf

• Infected seedlings often die, and may be confused with seedling disease. In seedlings and young plants, cotyledons and leaves wilt, may turn necrotic, and even fall off the plant, resulting in bare stems.

• Diseased plants exhibit a continuous discoloration of the vascular tissue

· Seedlings of susceptible varieties often die and in mildly affected plants, lower leaves develop symptoms but plants survive, but with reduced vigor and noticeable stunting.

• Fusarium wilt occur more if root-knot nematode infection is present. Certain strains of the causal fungus only cause symptoms when plants are also infected with the root knot nematode. In those cases, galls are usually prevalent on lateral roots.

 

 

Transmission

·       The fungus sustains itself on the outer surface of roots of many crops and weeds and survives indefinitely in soil.

·       The pathogen is seed-borne in cotton, causing long-distance spread,

·      Is also spread whenever infested soil is transported on boots, farm equipment, in flood irrigation, etc.

 

Verticillium wilt

Organism: Verticillium dahliae

Disease development depends on host: variety, developmental stage, and plant density, pathogen: defoliating vs. non- defoliating type, aggressiveness, and inoculum density. Occurs in  cool, wet environments

Symptoms

• Initial root infections occur early; symptoms are more evident post-bloom

• Infected plants may be wilted or stunted

• Chlorosis or necrosis of leaf margins and interveinal areas may be

• Light to dark brown discoloration (speckled) is prominent in the vascular system

• Severe defoliation can result, leading to reductions in yield and fiber quality

        

Management of cotton wilt

• Proper identification of pathogen is required. Sample infected plants for presence of  Fov / Verticillium

• Use pathogen-free seed produced in disease-free fields at all times

• Assay soil for root-knot nematodes- Nematode risk level will influence management options-  Use of nematicides will indirectly lead to less Fusarium wilt damage. Control of the root knot nematode is important to managing Fusarium wilt caused by most genotypes of the causal fungus. Nematicides, root knot nematode-resistant varieties or both are often necessary in fields infested with the nematode

• Rotation to any crop other than cotton prevents an increase in the soil population of Fusarium but may not significantly reduce the number of spores in the soil. The fungus will sustain itself on the roots of most plants, including weeds (without causing any symptoms), and cannot be eliminated by crop rotation alone.

• Remove infected plants. Use disease resistant varieties/cultivars for several seasons to reduce wilt severity in subsequent years

• Any field operation that moves soil from one location to another can spread spores of the fungus and introduce it to other fields. Washing soil from equipment with pressurized water will help limit the spread of Fusarium

• Other containment options include restricting traffic in affected patches, destroying affected plants and surrounding non-symptomatic plants, and stopping irrigation of affected patches in order to prevent movement of infested soil.