Causal agent(s) and transmission
Dry rots of tubers are caused by fungi of the genus Fusarium in particular Fusarium sambucinum = F. sulphureum and Fusarium solani var. cœruleum.
Occasionally this disease may be observed at the time of harvesting, although it usually appears during storage and eventually damages the entire tuber.
Infectious soil and tubers can be both the source of inoculum and the means of propagation. These fungi, in their resting forms (chlamydospores), can also survive in storage areas and on equipment.
Fungi of the genus Fusarium especially F. solani, can multiply and live in the soil which is a primary inoculum source for this species of Fusarium.
Other species of Fusarium present in the soil, e.g. F. oxysporum, can cause in dry conditions tuber rot in the soil and poor emergence but also later leaf dehydration and plant wilting due to the production of fungal toxins which are transported in the vascular system. Such disease is enhanced by short rotations and hot climates.
Fusarium dry rot and wilt are present worldwide and their economic impact increases once potato production becomes mechanised and relies on machinery for harvesting, handling and grading.
For seed potato production, this fungi group may be a serious cause of quality degradation.
Symptoms on tubers
On the tuber surface, the infected tissues turn brown (photo 1) and shrivel (dehydration), and may even show concentric streaks sometimes with pinkish to whitish mycelium (photo 2).
When cut in half, the tuber reveals a brown rot which develops into the flesh, with an unclear limit between healthy and infected areas, and with the potential presence of internal cavities covered with mycelium (photos 3 and 4). The tuber may dry out gradually, eventually becoming hard and mummified.
Under humid conditions, Fusarium-infected tissues can be readily invaded by secondary micro-organisms, such as bacterial soft rot, which can cause an extremely rapid wet decay.
Dry rot caused by Fusarium spp. (photo 5, bottom tubers) is usually not as dry as the depressed rot caused by Phoma (gangrene), (photo 5, top tubers).
Risk factors
Fungi of the genus Fusarium are wound and storage pathogens, as they penetrate tubers through wounds occurring in the course of harvesting operations, packing or transport and they develop further during storage. Until such time as wounds are completely healed, they may develop rapidly.
The optimum temperatures for tuber infection lie between 15 and 25°C.
Dry rots are also more frequent when farm hygiene (cleaning and disinfection, destruction of waste, etc.) is limited.
Control
- Good farm hygiene (washing and disinfection of all facilities and destruction of waste, etc.).
- Harvest within three to four weeks after haulm destruction when tuber skin is mature but not too late to favour disease development.
- Check that harvesting, grading and storage equipment is well adjusted in order to limit or avoid any wounds.
- Eliminate any tubers carrying rot.
- Thoroughly dry newly harvested tubers to allow rapid healing of the wounds.
- For seed potato production, chemical treatment may be applied to cleaned tubers shortly after harvesting. In some areas, a mixture of two fungicides, thiabendazole and imazalil, is recommended as many isolates of F. sambucinum are resistant to thiabendazole. However, the latter is effective on other fungi and on susceptible isolates of F. sambucinum.