As the potato harvest in the United Kingdom progresses, there are increasing reports of widespread scab. The weather in the United Kingdom was mixed last week, with some cooler temperatures but also warm and muggy at times. Rain showers have been sporadic and widespread.

The East

In the East, good and often above average yields have been reported for the region with lifting ahead of previous years. Lifting of early varieties is underway and most crops are now burnt off, however, skin set is taking longer than normal.

Scab has now been reported as widespread on susceptible varieties and following heavy rain last week, localised waterlogging has led to some circumstances of bacterial rot. Good dry matters are continually reported.

The West

In the West, crops are coming on with generally good yields. Lifting of green top Maris Piper is underway, with the majority of burning off due to be completed within the next two weeks. Scab is reported as widespread but not altering demand at this stage. A small amount of localised blight has been reported and is being treating accordingly.

The South

In the South, early lifting is coming to a close, with varieties such as Marfona giving way to Estima and later varieties. Yields have been reported as good to date, however, scab is often present, especially on salads.

Weather conditions have not hindered field work with progress generally seen as ahead of normal.

Scotland

In Scotland, trade is starting to pick up with the early lifting of whites well underway. Crop development has been reported as very good with excellent quality tubers, however, numbers could be better.

The generally wetter weather in Scotland has led to fewer reports of scab than in other regions for most varieties with the exception of M Piper, which has proved more susceptible.
 

Common Scab

Common-Scab

Common scab is caused mainly by Streptomyces scabiei. It is an unsightly blemish disease that can affect any crop where tubers experience a dry surface during the critical stage of 3-6 weeks after tuber initiation.

Consequently it can be especially problematic where irrigation is not available.

The disease has little effect on storability and does not develop in store.

Symptoms

Lesions may be circular or angular and may coalesce into large irregular areas. Severity can range from sparse colourless, corky lenticels to dark brown, raised or pitted scabs covering the tuber surface.

Common scab might easily be confused with powdery scab. However, powdery scabs tend to be smaller, have a more round character and when coalescing they tend to merge as discrete eruptions rather than one large scab.

Conditions

It is promoted by light free draining soils and dry weather in June/July, when rapid tuberisation takes place.

Some cultivars of potato are more resistant to the scab (Melody, Cara) than others but none are immune.

Control

To prevent common scab developing in dry conditions you should irrigate every 4 to 6 days for 3 to 4 weeks after tuber initiation.

The soil structure should ideally be fine to ensure contact between damp soil and tubers. Avoid increasing the pH of soil, plant clean seed and maintain long.

Recent UK research on Common Scab Control