Preliminary study on the effect of horseradish extract on potato common scab

 

Introduction

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 variability may explain why the production practices recommended to control scab have been so inconsistent. Once scab is introduced into the soil, it will survive indefinitely, even in the absence of potatoes.

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 favorable for scab development. A closely related species, S. acidiscabies, is acid tolerant and can produce lesions when potatoes are grown 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. The pathogen produces thaxtomin, a toxin that is essential to produce scab lesions.

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 a cm. Lesions also vary in size and shape. They may be few and scattered or may cover most of the tuber surface.

Many factors influence the incidence and severity of scab: soil pH, soil organic matter, bacterial species and strain, soil texture, soil temperature and soil moisture. The incidence and severity of scab varies from year to year and from field to field. In Ontario, there is usually more disease in sandy soils than in heavier soils. Warm, dry soils favor scab as does the addition of lime.

Management practices recommended to control scab rarely give consistent results. For example, some growers report 70 to 90 % control with fumigation in the fall using Chloropicrin while others report no control at all. There are varieties resistant to scab, but many of the varieties consumers demand are very susceptible to scab. A control method that is effective, reliable and affordable would be welcomed by potato growers.

2009 Scab Trial

This past season, I evaluated several materials to reduce scab incidence including horseradish extract. A potato grower from Ontario--Wally Vanderstelt - suggested that horseradish might have an effect on the scab pathogen. His suggestion was based on many years of field observations. The quickest way to test this was to extract horseradish juice, mix it with scab infested soil and grow potatoes in pots.

Material and Methods

Treatments

The trial consisted of two treatments:

• Treatment 1: Horseradish extract from the variety Big Top Western (3 pots )
  
• Treatment 2: Check (3 pots)

The pots used were 30 cm in diameter and 40 cm high. Holes were drilled in the bottom of the pots for drainage, and each pot was filled with heavily infested soil collected from a scab research plot. The soil moisture was 60% of water holding capacity.

Horseradish treatment

A juice extractor was used to extract 1.2 L of horseradish juice from roots. Then, 300 mL of horseradish juice was mixed thoroughly with the scab infested soil in each of 3 pots. The fiber remaining after the juice extraction was also mixed with the soil.

Check treatment

Instead of juice, 300 mL of water was mixed with the soil in each of the 3 check pots. .

The pots were stored at 150C for two weeks (May 15th to May 29th). On May 29, fertilizer was added to all the pots and a single, whole Yukon Gold seed tuber was planted in each pot. The pots were left in a corner of the scab research plot in Alliston and were not irrigated. The tubers were checked for scab on August 20-22

Results and Conclusion

Table 1 shows the results of this trial.

Table 1. Average Scab incidence

Rep #	Horseradish Treatment	Check
Rep 1	0.2%	30%
Rep 2	0%	25%
Rep 3	0%	40%

 

As expected, the tuber set was lower and the tubers were smaller on the potato plants grown in pots compared to the potatoes grown nearby in the scab field trial. Only one tuber in one pot from the horseradish treatment had scab, and that was a negligible surface lesion. Scab incidence in tubers grown in the check pots was high.

Further laboratory and field studies are needed to confirm that the results observed in pots can be applied in the field:

• Identify the isothiocyanates in horseradish and test them to determine if any are effective against scab.
• More practical field application methods should be evaluated e.g. incorporating crushed roots into fields prior to planting.

As mentioned before, a scab control method that is effective, reliable and affordable would be welcomed by potato growers. Horseradish might help to control this formidable foe?

Please note that these results are preliminary in nature; the use is not registered or being recommended by OMAFRA at this time.

Horseradish juice

Horseradish trial