Wednesday, 28 July 2010

Dickeya solani, no ordinary blackleg

Last year many of us heard for the first time about the aggressive form of Dickeya, provisionally named Dickeya solani. First seen in the Netherlands and now found in several West European countries, a handful of cases were reported in England last year. It has always been a complicated business to get to grips with the complexity of symptoms (blackleg, stem rot,wilting, tuber wet rots) associated with a complexity of causal organisms. It is even more complicated when scientists change the names of these organisms. Until now we have been familiar with blackleg, caused by Pectobacterium atroseptica (old name Erwinia carotovora supsp. atroseptica) and blackleg-like symtoms, often accompanied by wilting, caused by a group of species known as Dickeya (old name Erwinia chrysanthemum). Until recently outbreaks in the UK have been found to be caused by Dickeya dianthicola and really only found in crops grown from Dutch seed or crops originating from Dutch seed. The new form has been described as more aggressive, more likely to infect at lower levels and more likely to transmit from plant to plant along rows and even across rows.

This season I was shown a crop of Marfona, which showed blackleg-like symptoms, but with a severity that immediately suggested that this was no ordinary blackleg. The stem rots often affected every stem of a plant, completely arresting the development of this plant shortly after tuber initiation. At least 40% to 50% of plants were affected and the field showed large areas of bare land as a result. Losses may well be in excess of these figures, since the performance of the remaining plants, even if they developed no symptoms later, would be badly affected by the uneven crop stand.

The seriousness of this disease is such that growers would do well to insist on UK, Safe Haven sourced seed. The alternative is to have seed tested. However no testing scheme can give complete assurance because a sample is never going to give 100% accuracy. FERA estimate that whith 3 samples of 200 tubers, the confidence of detecting 0.5% vascular infection, is 95%. A 100 tuber sample will only give a broad indication of high, medium and low risk. The bacteria can apparently infect plants through the roots. Transmission from diseased plants also appears to take place through the vascular system (for Pectobacterium atroseptica this is usually through the breakdown of the seed tuber and contamination of tubers through the soil).