Purple top roll of potato in France
In July 1974, growers and technicians informed the Regional Plant Protection Station of “Nord and Picardie” that numerous potato fields in the Departments of Nord and Pas-de-Calais showed plants with purple rolled shoot ends.
Description of the symptoms :
The symptoms always appeared at the end of the shoots but they were different according to the potato varieties : Bintje showed a dark purple top roll while for Ker Pondy the extremities had a caracteristic purplish pink ; on other varieties as Ackersegen a yellowing of the shoots could be noticed. In each case the shoot ends became caracteristically stiff, the leaves were smaller and there was an upward leaf roll.
Sometimes, proliferation of axillaris occur and there is a thickening of stalks at the knots. Small aerial tubers may show some leaflets on the different knots at the axil of the leaves, particularly on Ker Pondy, in the absence of black scurf. On Ackersegen and Panther apparently healthy shoots appeared after the diseased ones.
None of the considered alternatives (burning, herbicides, deficiencies, fungi, known viruses) being met, it was suggested that the symptoms might be due to a new virus or mycoplasma. The observation of leaf roll on affected plants seemed to indicate the presence of a leaf roll-like virus which sometimes occurs in France. Mr SPIRE, virologist at INRA1) showed that these symptoms were quite different from those actually described for potato viruses.
Plants were sent to the mycoplasma laboratory and the first observation revealed symptoms which are typical for mycoplasma ; investigations are ongoing.
It is also important to determine the vector of the disease. The leafhopper Hyalesthes obsoletus Sign mentioned as Stolbur vector in Central Europe is also found in France, but its capability as vector has not yet been demonstrated.
Climatic conditions, specially those prevailing in the previous year, have an important effect on vector populations entailing epidemics. This is what France was apparently experiencing in 1974.
1) INRA : Institut National de la Recherche Agronomique.
Plant Protection Service, France (1974-10-11)