Causal agent(s) and transmission
The agent causing black dot on potato tubers is the fungus Colletotrichum coccodes, besides damaging tubers, it can also affect roots, stolons and stems.
The sources of infection are essentially plant debris (potato and tomato vines), infected weed hosts (Solanum nigrum, Physalis peruviana, Datura stramonium), and microsclerotia carried by seed tubers or in a free state in the soil.
Microsclerotia that are present in the soil can be infectious for more than 2 years.
Black dot is present in many areas of the world but its direct effect on yield is usually alleviated by adequate fertilisation and good irrigation management.
However, in favourable soils and when fresh ware tubers are washed before selling, black dot is a surface-blemishing disease which may downgrade marketable yield and thus cause economic loss.
Symptoms on foliage
Under disease pressure, the symptoms appear sporadically, and mainly in hot summers.
The typical symptom is premature death of the haulms with drying of the foliage extending from the top to the bottom of the plant: the leaves turn yellow and curl upwards (photo 1). These symptoms are similar to those of Verticillium wilt.
At the end of the growing season, the base of the stems and the underground parts of the plant display signs of typical infection: early destruction of the roots, detachment of the root envelope and numerous black dots on the base of the stem (microsclerotia) (photo 2).
Symptoms on tubers
Lesions on tubers caused by fungal infection can be observed (photos 3 and 4): irregular and diffuse light to dark greyish surface areas with or without the formation of fructifications (microsclerotia) with setae when tubers are exposed to moisture (photos 5 and 6). Fructifications due to black dot (microsclerotia) are larger than for silver scurf (conidiophores) (photo 7).
When tubers have been subjected to cold temperatures, development of deep sunken lesions are sometimes associated with dry rot of the underlying tissues (photo 8).
The optimum temperature for this fungus is between 25 and 30°C, which accounts for its more rapid development in the summer periods, although it is quite capable of developing at much lower temperatures, especially during storage.
The disease is often associated with light sandy soils, inappropriate fertilisation and short rotations.
Currently, few, if any, cultivars have resistance to this disease. However, thin-skinned cultivars appear to be more susceptible than thicker-skinned. Consequently, the following measures are recommended for limiting black dot disease:
- use long rotations and leave at least 4 years between two potato crops;
- plant healthy seed tubers;
- seed or soil treatment can limit the disease if they are well applied and approved;
- avoid growth stress, ensure adequate water supply, with adapted fertilisation and appropriate manure and irrigation;
- limit the inoculum by destroying crop waste and infected haulms and by efficient weed control, especially for Solanum nigrum, Physalis peruviana and Datura stramonium;
- harvest no later than 3-4 weeks after haulm destruction;
- take basic precautions during storage: ensure thorough drying at harvesting and after washing; avoid condensation on tubers, especially after storage;
- maintain low storage temperature, below 5°C, as early as possible as the development of the fungus is significantly slowed down below this temperature.