November 6, 2005
From: American Phytopathological Society, Plant Disease Notes [edited]

Recent studies on Thecaphora solani

Potato smut caused by _Thecaphora solani_ (EPPO A1 list) is a serious disease of potato, only present in the Andean region of South America, in Panama, and Mexico. It affects various cultivated species of _Solanum_ (_S. tuberosum_, _S. andigenum_, _S. stoloniferum_) and other _Solanaceae_ (e.g. _Lycopersicon_, _Datura stramonium_). Up until now, the fungus has not been cultured or characterized in vitro.

In Chile, _T. solani_ was first reported in 1974 in region IV (region of Coquimbo), where it has established in many soils causing significant yield losses (more than 90 percent) in potato crops. The disease was then reported in region VIII (region del Bio-Bio). In 1996, potato smut was discovered more than 1000 km further south from its initial finding, in region IX (region de la Araucania) near Carahue. As this represented a very serious threat to seed-potato production areas, efforts were made to try to eradicate the disease and to study the basic biological aspects of the pathogen and the disease.

Therefore, studies were done in Chile to culture _T. solani_ in vitro, and to characterize the fungus by molecular methods. For the first time, _T. solani_ was cultured on solid and liquid growing media. But this remained a laborious and difficult task, as only 7 of the 26 isolates studied could be cultured. This was mainly due to the low germination capacity of _T. solani_ teliospores.

The sexual cycle of the pathogen was completed under laboratory conditions. 1st attempts to reproduce the disease under controlled conditions were also successful. Artificial inoculation of mycelia to potato explants grown in vitro (_S. tuberosum_ cv. 'Desiree') induced a gall. Molecular analysis also confirmed the initial classification of _T. solani_ in the genus _Thecaphora_ (it had been suggested at one point that it belonged to the genus _Angiosorus_).

Source:
Andrade O, Munoz G, Galdames R, Duran P, Honorato R (2004) Characterization, in vitro culture, and molecular analysis of _Thecaphora solani_, the causal agent of potato smut. Phytopathology, 94(8), 875-882.

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[Potato smut [PS] is 1 of the most destructive fungal diseases affecting potato (_Solanum tuberosum_) in the Andean region of South America, producing yield losses of up to 85 percent. It is also recorded from areas of Panama and Mexico, but there are no reports of PS elsewhere worldwide. PS affects various cultivated species of _Solanum_, such as _S. tuberosum_, _S. andigenum_ (Andigena), and _S. stoloniferum_. It also has been reported on tomato (_Lycopersicon_), and on some common weeds such as _Datura stramonium_. _T. solani_ survives in soil or in tuber debris, and its spores are thought to be long-lived, though it is not known how long they are viable. Pathogen incidence is increased in the absence of crop rotation and is favored by high humidity and saline soils.

Field observations indicate that seed tubers transmit the disease. PS has a very low natural dispersal potential. The most likely way in which PS could enter un-infested areas would be with potato tubers. It has been intercepted by the USDA in 52 consignments of tubers of _Solanum tuberosum_ and _S. stoloniferum_, almost all from Mexico, since 1946 (USDA Lists of Intercepted Plant Pests). Soil from infected areas could also introduce the fungus. PS is reported to be serious, accompanied by losses of up to 80 percent in the very susceptible cultivar Peruanita.

In Peru field trials, methyl bromide and dazomet, used as soil dis-infestants against _T. solani_, reduced the weight of hypertrophied tubers considerably (but did not eliminate the fungus).
Susceptibility to infection varies, some cultivars being resistant.
Resistance screening is actively being pursued at CIP (International Potato Center) in Peru.

General control measures recommended are: use of resistant cultivars, planting of smut-free seed potatoes, long rotation, elimination of the weed _Datura stramonium_ (also reported as a host), and removal of smutted galls.

Links:
<http://www.eppo.org/QUARANTINE/fungi/Thecaphora_solani/THPHSO_ds.pdf>
<http://www.apsnet.org/phyto/SubscriberContent/2004/0617-02R.pdf>
- Mod.DH]