Irrigation and Common Scab

The role of irrigation scheduling on the incidence of common scab on potato tubers has been extensively studied since the mid 20th Century.

Infection — Common scab (Streptomyces scabies) infects lenticels or pores of growing potato tubers. Lenticels are found in the internodes, the area between bands of eyes. Eyes mark the nodes on the potato tuber. The scab forms as a wounding response to infection by common scab, a bacteria with some fungal properties. The susceptibility of the tuber and severity of the scab, surface to pitted, depends on the variety since there is a genetic resistance component in many varieties. Research reviewed here was conducted on Russet Burbank, Irish Cobbler, Majestic, Maris Piper, and Kufri Chandramukhi.
During the course of tuber diagnostics, many times tubers were observed with scab areas that matched drought periods and pictures of scabby tubers in these published research papers suggesting an irrigation scheduling problem.

Drought

The amount of scab on a tubers surface is directly related to the length of time that potatoes are deprived of irrigation. Figure 1 (modified from Lapwood & Hering, 1968) shows the percent of tuber surface with scab in relation to the length of a drought period beginning on June 15. With a drought period from 6/15 to 6/20, five days, the amount of scabby surface increased from zero to over 20% making tubers unmarketable. A 10-day drought period caused over 40% of the surface area to be scabby. (Dates are based on a 5/1 planting).

When a drought period starts also affects the amount of surface that's scabby. Figure 2 (modified from Lapwood & Hering, 1970) shows the effect of a 7-day drought period started on different days in June and July (based on a 4/30 planting). Drought periods started in June caused the largest amount of scabby surface on tubers. Only when the drought period started after tubers had more than 6 nodes (eye number) was the infected surface area less than 5%. Note later internodes are shorter than ones formed early in the tuber growth.

Early drought periods affect the first internodes while later periods affect the later formed internodes. This is demonstrated in Figure 3 (also modified from Lapwood & Hering, 1970). It is key to note that earlier formed internodes (solid bracket) are longer than later ones (dashed bracket). This couldn't be depicted on the figure. Infection of the second to the fifth internode (stolon end) occupies most of the tuber surface while infection of the last five internodes (bud end) results in less than 15% of the tuber surface to be scabby. The amount of surface that's scabby is directly related to drought periods in relation to tuber growth and internode development.

If irrigation starts by the time the number of internodes on tubers is less than three, then there is little scab-infected area. But, if irrigation doesn't start until there are 4-5 internodes, then the first two internodes will be scabby (Figure 4, modified from Adams & Lapwood, 1978). If there are seven internodes present at the start on irrigation, then the first four internodes are scabby resulting in over half the tuber's surface area to be scabby.

Soil Moisture

So the period of greatest scab damage in terms of surface area is during the early bulking period, around and shortly after flowering. How much soil moisture is needed to keep the number of scabby tubers and the degree of scab low? Looking at Figure 5 (modified from Singh & Singh, 1981), one can see that 90% field capacity is needed to decrease the percent of scabby tubers to below 20 and at least 75% to lower the scab index to 5. (Scab index is based on infected surface area and scab type, see Erickson, H.T. 1960. Am. Potato J. 37:18-22.)

How long does the soil moisture have to be at 90% field capacity to reduce scab? Figure 6 (modified from Davis et al. 1974) shows that a minimum of six weeks is required to reduce the number of tubers with less than 5% scabby surface to 5%, US grade A standard marketable level. To achieve more than half the Russet Burbank tubers to be scab-free, nine weeks at 90% field capacity was needed. Six weeks corresponds to the start of the tuber bulking period to past the mid-point.

Summary Highlights

1. Early-bulking, low, soil moisture results in scab near the stolon end and a greater tuber surface area infected by scab.

2. Later low soil moisture results in scab near the bud end and less surface area is affected by scab.

3. New internodes, the area in between eye bands, get scabby during low soil moisture periods.

4. On sandy soils, the critical period starts 2-3 weeks after tuber initiation. This corresponds to about row closure or early flowering for most varieties.

5. Every 5-day drought period corresponds to 2-4 internodes infected by scab.

6. The sensitivity of an internode to scab infection is its second week after formation.

7. Soil needs to be at 90% field capacity during early and mid bulking to effectively suppress common scab.

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Figure 2:

Figure 3:

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Figure 5:

Figure 6:

References:

• Adams, M.J. and Lapwood, D.H. 1978. Infection by common scab of potato tubers growing in wet and dry soils. Annals of Applied Biology 90:335-343.
• Davis, J.R., McMaster, G.M., Callihan, R.H., Garner, J.G., and McDole, R.E. 1974. The relationship of irrigation timing and soil treatments to control potato scab. Phytopathology 64:1404-1410.
• Lapwood, D.H. and Hering, T.F. 1968. Infection of potato tubers by common scab during brief periods when soil is drying. European Potato Journal 11:177-187.
• Lapwood, D.H. and Hering, T.F. 1970. Soil moisture and the infection of young potato tubers by common scab. Potato Research 13:296-304.
• Singh, N. and Singh, H. 1981. Irrigation -- a cultural practice to control common scab of potato. Journal Indian Potato Assoc. 8:35-36.