Candidatus phytoplasma solani

(Stolbur)

Since  many years ago, tobacco farmers in different production regions have observed leaf yellowing in fields usually occurring in July and August. First the yellowing was attributed to a possible nutritional disorder but this speculation  was quickly revised. Indeed, the morphological abnormalities observed in buds can be caused by a phytoplasma (formerly mycoplasma or MLO = mycoplasma like organism). Transplanting the apex of diseased plants on healthy rootstocks, use of immunofluorescence with specific monoclonal antibodies, and the use of molecular biology methods (PCR) confirm the presence of one of the phytoplasmas causing stolbur in Solanaceae, in this case in France it is .
 
The disease starts as a  leaf yellowing, which is first interveinal, occuring most frequently on older leaves or middle leaves and gradually covers the whole plant (figures 1-3). In dark tobacco leaves, veinal and interveinal necrosis occasionally accompany a slight chlorosis of the lamina (figure 4); the observed syndroma may be confused with that caused by certain necrotic strains of potato virus Y (PVY), see the fact sheet.
 
If plants are infected early by the phytoplasma, and especially in dark tobacco the discoloration is such that the lamina becomes completely white. This symptom may be the cause of what tobacco farmers in the past sometimes referred to as "white feet" or "white tobacco". This disease, which occurred on the particular variety of Nijkerk in the Lot region (France) has many features in common with the Stolbur:
- Yellowing and whitening of the leaves beginning during or after topping;
- Isolated diseased plants in the field ;
- Sterile floral parts when present, hypertrophy of ovaries in particular;
- Leaves of poor quality, drying inadequately and tending to mould more easily.
It is not unreasonable to think that this disease, which was earlier attributed to plants with a limited root system subjected to abrupt climate changes, is in fact due to Stolbur.
 
The abnormal colour is certainly not the most characteristic symptom of the disease, and it might have been the source of much confusion with the symptoms caused by nutritional disorders.
 
The most typical symptoms are observed on the uninhibited buds. The young shoots have a reduced growth, much shorter internodes, chlorotic leaves of reduced size and the leaf tips tend to curve downwards (figure 5). On some plants, the proliferation of numerous short and stiff branches at the apex gives the plant a tufted growth appearance (figure 6). For other types of symptoms, you can also see the following link (Abnormal leaf size and shape). To complete information on this disease, please consult the Stolbur Solanaceae fact sheet.
 
Note that the harvested leaves are of poor quality. In addition they mould easily during drying, their tissues are poorly coloured and boardy (see the fact sheet of Curing barn diseases).