Fusarium spp. is a group of soil-borne fungi with many different species. Fusarium is widespread and can infect a range of host crops. Many species are considered weak pathogens and can only infect wounded or stressed host plants.
Three pathogens can be distinguished:
- Fusarium oxysporum (Fusarium wilt): Many different hosts, potato (Solanum tuberosum), most vegetables and ornamental crops, tomato (Solanum lycopersicum) tulips (Tulipa), chrysanthemum, banana (Musa sp.)
- Fusarium graminearum/Gibberella zeae (Fusarium head blight or scab): Cereals, soybean (Glycine max), tobacco (Nicotiana tabacum), lupine (Lupinus sp.)
- Fusarium sambucinum/Fusarium sulphureum (Dry rot of potato): Potato (Solanum tuberosum)
Life cycle and appearance of Fusarium spp.
Fusarium is a group of soil-borne fungi with many different species. Fusarium is widespread and can infect a range of host crops. Many species are considered weak pathogens, which can only infect wounded or stressed host plants.
Fusarium oxysporum has many different ‘formae speciales’ which are each selectively pathogenic on a limited number of crops. Even within one crop, different 'formae speciales' may occur and cause different symptoms. This species is the only Fusarium that actually grows inside the vascular system of the host plant and spreads upwards inside the plants. The other species are dispersed upwards on the outside of the plant.
Most Fusarium species only make asexual spores. Some also produce ascospores. In general, the life cycle of different Fusarium species is similar. Fusarium overwinters for many years in the soil, and on crop residues of infected plants, as chlamydospores (thick walled mycelium cells) or mycelium. Survival is also possible on seed, greenhouse structures, tools and machinery. Primary infection is either seed-borne or takes place as infection of the roots at the root tip, or in small wounds, for example where lateral roots branch off the tap root.
F. oxysporum grows inside the roots towards the foot of the plant and enters the vascular system. Inside the vessels, the fungus produces mycelium and many spores (microconidia), which are transported upwards. At the same time, the plant produces gum as a defence mechanism to stop the fungus from spreading. This gum blocks the xylem and causes the wilting which is the main symptom of this Fusarium species. Higher up in the plant, the microconidia germinate and infect the surrounding tissue. On infected plant parts, the typical boat-shaped pink macroconidia are produced, which may be dispersed further.
Fusarium graminearum (Gibberella zeae) overwinters as chlamydospores and ascospores in crop residues. Primary infection is from this inoculum, or from infected seed. Infected seed doesn’t germinate well and seedlings die, causing irregular crop density. The foot of the plant is infected either because the seedling was infected, or by direct infection of the root by overwintering inoculum. On the infected root, conidia are produced, which are the source of further spread of the disease. F. graminearum/G. zeae primarily infects the older, lower leaves. The spores are dispersed upwards to the ears by rain splash, or wind. On the infected ears, the typical pink spores are produced, which may further spread the disease. Ascospores of F. graminearum/G. zeae germinate between 4-35°C, with an optimum between 25-28°C. Germination only occurs at RH above 80%. Warm (25-30°C) and humid (RH above 85%) conditions enhance infection.
Fusarium sambucinum is a general soil inhabitant. The fungus infects tubers which are damaged at harvest. It may also infect tubers from inoculum present on tools and in storage sheds. Disease is enhanced by the presence of nematodes which cause wounds on the tubers through which F. sambucinum can enter. The optimum temperature for infection is 10-20°C, and 15-20°C for the development of rot.
Damage symptoms of Fusarium spp.
Fusarium fungi cause vascular wilt, root rot, foot and stem rot, leaf lesions, fruit rot, head blight in cereals, and post-harvest decay.
Fusarium oxysporum (Fusarium wilt) is the species that causes vascular wilt. Initially, the leaves turn yellow and wilt, mostly on one side of the plant. Finally, the whole plant wilts. Other symptoms are brown discolouration of the xylem vessels, which can be seen when the stems are cut. In banana, whole plantations may die, and the soil may not be suitable for planting for many years subsequently.
F. graminearum/Gibberella zeae (Fusarium head blight or scab) (anamorph/teleomorph) causes seedling damping-off. On the foot of the stem, brown discolouration occurs. Infected glumes and kernels become white, due to dehydration, and later turn pinkish-red as spores are formed. Ear infection decreases the baking quality of wheat and brewing qualities of barley. Furthermore, mycotoxins are produced, of which DON (deoxynivalenol) is the best-known. The mycotoxins are toxic to mammals so maximum levels have been determined.
Fusarium sambucinum (a.k.a. F. sulphureum) causes dry rot of potato. The rot is dark and wet at first. The lesions are irregular and sharp-edged. In cavities, pink sporulation becomes visible.
Damage symptoms of Fusarium spp.
