Features Agronomy Tillage
Best bets against common scab

Common scab occurs in most potato growing areas of the world. The causal organism
Streptomyces scabies has many strains, and some strains of this bacterium
are more aggressive than others. This strain variability may explain why the
practices recommended to control scab have been so inconsistent. The bacterium
survives in the soil for long periods of time even in the absence of potatoes.
Common scab occasionally affects other crops: carrots, beets, radish, turnips
and parsnips.

There are two other species of Streptomyces, S. acidiscabies
and S. turgidiscabies, that cause common scab on potatoes, but these
species have a more limited geographic distribution. All three pathogens produce
thaxtomin, a toxin that is essential to produce scab lesions.

Scab bacteria are introduced into healthy fields by planting infected seed,
by spreading contaminated cattle manure, by spreading infected grade-out tubers
in clean fields and by carrying infested soil on farm equipment.

A soil pH from 5.5 to 7 is most favourable for scab development. S. acidiscabies,
however, is acid tolerant and can infect tubers in soils with pH below 5.

Scab does not reduce yield but the lesions affect tuber quality, rendering
the potatoes unmarketable. Most scab infections take place at tuber initiation.
The pathogen enters newly forming tubers through immature lenticels stimulating
excessive cell growth and causing cell death.

Scab symptoms vary greatly. In mild cases, there is only netting on the skin.
More severe infection results in superficial or raised areas of rough, corky
tissue. Pitted scab has cavities that may be as deep as one centimetre. Lesions
also vary in size and shape. They may be few and scattered or may cover most
of the tuber surface. As tubers increase in size, lesions expand and scab severity
increases. The types of lesions that develop are probably related to the aggressiveness
of the Streptomyces strain, the time of infection and the environmental
conditions.

The incidence and severity of scab varies from year to year and from field
to field. The incidence of the disease is generally greater in sandy than in
heavier soils. Warm, dry soils favour scab development. The addition of lime
usually increases scab severity.

Because too many factors influence scab incidence and severity (soil pH, organic
matter content, bacterial species and strain, soil texture, soil temperature,
soil moisture), management practices recommended to control scab have given
inconsistent results. No single method – except resistant varieties –
has proved reliable and effective. It seems that control methods should be custom
tailored for individual fields.

Management practices recommended to reduce scab incidence and their efficacy
are presented in Table 1. Also included in the table are promising control methods.

All researchers agree that at the present time, genetic resistance is the most
reliable, cost effective control strategy for common scab. Susceptible varieties
such as Yukon Gold, Envol, Nordonna to name just a few, should be grown in scab-free
soil and in scab infested fields, grow varieties that are tolerant to scab.

Table 2 presents the results of a scab trial conducted in 2005 in Alliston,
Ontario. One hundred varieties/lines were grown in a heavily infested plot.
Scab rating was done at harvest. -30-

*Eugenia Banks is the potato specialist for the Ontario Ministry of Agriculture,
Food and Rural Affairs.