Fusarium dry rot is caused by a number of Fusarium species but is generally associated with Fusarium sambucinum, F. solani, F. culmorum and F. avenaceum.
Infections generally begin at wound sites. Once infection occurs, it slowly enlarges in all directions. The skin over the infected area sinks and wrinkles, sometimes in concentric rings, due to the fungus drying out the contents of the tuber (Figure 1).
Internally, infected areas are light brown to black as the fungus kills the cells of the tuber. Internal cavities created by dry rot infections generally contain fungal mycelium of various colours (Figure 2).
The infected areas usually remain dry but at high moisture levels or humidity bacteria invade and cause foul smelling wet infections. If infected areas are not removed, the tuber can completely rot and shrivel.
In the field, symptoms include variable seed emergence and differences in plant size. Fusarium species can also cause a wilt which includes stunting of growth, chlorosis of leaves and wilting of lower leaves.
Fusarium dry rot is both seed and soil-borne and is present in most potato growing areas. Spread is associated with damage through seed cutting, grading or harvesting. Wounds created during these processes allow the Fusarium fungi to enter the tuber and spread.
Temperatures of 15 to 20°C and high relative humidity aid the growth of fusarium dry rot. Lower temperatures and humidity retard the fungus but dry rot development continues even at the lowest storage temperatures (4°C).
Seed tubers may be infected prior to shipment but not exhibit symptoms until during or after transit. Cultivars differ in their susceptibility to dry rot.
Fusarium species can survive in the soil for a long time as either survival spores or on decaying plant material. Untreated wounds or cut seed are susceptible to soil-borne infection. Soil attached to tubers at harvest will generally contain spores that can lead to infection during storage.