Dry rots
Fusarium roseum var. sambucinum and F. solani var. coeruleum,
Fusarium sp.
Causative agent and transmission
Late blight (fusarium) of tubers is caused by fungus of the genus Fusarium (particularly Fusarium roseum var. sambucinum and Fusarium solani var. coeruleum). This disease may be observed exceptionally after harvesting, but it appears more generally during storage and leads to destruction of the tuber.
The contaminated tuber and soil carry the fungus and are vectors for propagation; owing to its conservation in the form of chlamydospores, the fungus can also remain in storage premises and on equipment.
Fusarium can multiply and remain in the soil, thus providing abundant primary inoculum.
The optimum temperature for infecting the tubers is in the range 15-25° C.
Description of symptoms on crops
(clic on photos to enlarge)
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On the surface, the infected tissues turn brown and shrink (dehydration), they can even show concentric threads, sometimes decorated with whitish mycelian.
Cutting the tuber reveals a brown rot developing inwards which is associated with internal cavities lined with mycelium. The tuber may gradually dry up until it becomes hard and "mummified". On the other hand, under damp conditions, there are additional bacterial attacks which cause soft rot.
Fusarian rot (photo below: tuber on left) is generally less dry than the "thumbprint"-type rot (tuber on right) caused by Phoma
Dry rot caused by Fusarium spp. (tuber on the left) is usually not as dry as the depressed rot caused by Phoma (gangrene, tuber on the right)
Control
- Avoid damaging the tubers when handling;
- Harvest within three to four months after haulm defoliation;
- Eliminate any tubers carrying rot;
- Dry the tubers thoroughly on harvesting and encourage healing of wounds;
- Treat shortly after harvesting with a fungicide containing a thiabendazole + imazalil base to control all strains;
- General hygiene on the farm (washing and disinfecting the premises and equipment, elimination of waste, etc.).
