Common Scab of Potato
Phillip Wharton1, Jarred Driscoll2, Dave Douches2, and William Kirk1
of Plant Pathology, 2Department of Crop & Soil Sciences, Michigan State University
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Of the more than 400 identified species in the genus Streptomyces only a fraction are considered plant pathogenic. Common scab may be caused by several soil dwelling plant pathogenic bacterial species in this genus. These include S. scabies and S. turgidiscabies. In particular, S. scabies has been well documented as causing scab lesions. Streptomyces scabies infects a number of root-grown crops including radish (Raphanus sativus), parsnip (Pastinaca sativa), beet (Beta vulgaris), carrot (Daucus carota), as well as potato (Solanum tuberosum). The disease occurs worldwide wherever potatoes are grown. Although scab does not usually affect total yields, since the marketplace for potatoes is quality driven, the presence of scab lesions, especially those which are pitted, significantly lessens the marketability for both tablestock and processing varieties.
The symptoms of common potato scab are quite variable and are manifested on the surface of the potato tuber. The disease forms several types of cork-like lesions including surface (Fig. 1), raised (Fig. 2), and pitted lesions (fig 3). Sometimes surface lesions are also referred to as russeting, particularly on round whites (Fig. 1), as the general appearance resembles the skin of a russet-type tuber. Pitted lesions vary in their range of depth, although on average they extend 3 to 4 mm deep. The type of lesion formed on a tuber is thought to be determined by a combination of host resistance, aggressiveness of the pathogen strain, time of infection, and environmental conditions.
Scab symptoms are usually first noticed late in the growing season or at harvest. However, tubers are susceptible to infection as soon as they are formed. Lesions start out as small, brownish spots, which enlarge into water-soaked circular lesions within a few weeks of infection. These circular lesions may coalesce forming large scabby areas (Fig. 3). Scab is most severe when tubers develop under warm, dry soil conditions with a soil pH above 5.2 (Fig. 4).
Common scab is greatly suppressed in soils with a pH of 5.2 or lower. However, tubers grown in acidic soil may develop scab-like lesions. This may be due to acid scab, a similar disease to common scab caused by the related pathogen S. acidiscabies. The acid scab pathogen can grow in soils with a pH as low as 4.0. Acid scab and common scab are hard to differentiate as lesions caused by S. acidiscabies, are similar, if not identical, to those caused by S. scabies.
Common potato scab is an efficient saprophyte that can over-winter either in soil or on the surface of tubers and crop residues. The pathogen is spread from one location to another by splashing water (irrigation or rain), wind, and on seed tubers and farm equipment with leftover soil residue. Most if not all the soils where potatoes are grown in Michigan have a resident population of S. scabies. The population may be reduced by rotation with grain crops or other non-hosts but cannot be eliminated entirely since it reproduces to some extent on organic matter in the soil.
Streptomyces scabies has branched mycelium. Its sporogenous hyphae develop into corkscrew-like spiral chains with cross walls that eventually constrict and break off into individual spores. As spores mature they develop a gray or melanized pigmentation. When a spore comes into contact with a suitable host it will germinate and the infection process begins.
The optimum temperature for infection of potato tubers by S. scabies is 68 to 72°F, but the pathogen can attack tubers in soil within a wide range of temperatures from 50 to 88°F. Infection usually begins with the onset of tuberization. The pathogen primarily invades lenticels, but will take advantage of any open wound on the surface of the potato tuber. After penetration the pathogen grows through up to three peridermal cell layers causing the cells to die. The bacterium then feeds on them saprophytically. The pathogen also secretes a compound that promotes rapid cell division in the living cells surrounding the lesion. This causes them to produce several layers of cork (suberized) cells that isolate the bacterium and surrounding tuber cells. As the tuber cells above this suberized layer die, the pathogen feeds on them. As the suberized layers are pushed out and sloughed off the pathogen grows and multiplies in the additional dead cells which results in the development of the scab lesion. This cycle may occur several times throughout the growing season, and therefore will produce a larger lesion. Lesion size will also vary depending on when infection occurs. Generally, the earlier a tuber becomes infected, the larger the lesion.
Monitoring and control
No single measure provides effective control of scab. However, the disease can be managed using an integrated approach that combines the use of host resistance and cultural control methods. Chemical control methods have met with limited success.
There are several factors which are known to influence scab disease severity. These are soil moisture, soil acidity, soil type and amendments and crop rotation. Soil moisture during tuberization has a dramatic effect on common scab infections. Irrigation has been used as a way to manage this disease since the early 1920’s. Maintaining soil moisture levels near field capacity during the 2 to 6 weeks following tuberization will inhibit infection. However, irrigation may not always be the most practical, especially for soils with low water holding capacity. Furthermore, other disease problems such as Pythium Leak and Pink rot, may be aggravated by excessive irrigation.
Acidic soils, with a pH level below 5.2 can also significantly reduce the severity of common scab and potatoes are commonly grown in soils with a pH of 5.0 to 5.2 for control of common scab.
Soil amendments including manure, lime, and cover crops have produced inconsistent results for the control of scab. Streptomyces are generally involved in the decomposition of organic matter and therefore thought to be stimulated by its presence. Thus, if possible it is best to avoid light textured soils and those with high levels of organic matter.
Crop rotation is important in the control of common scab as it reduces the levels of inoculum in potato fields. However, S. scabies can survive for many years in the absence of potatoes due to its ability to live saprophytically and infect other plants. It has been reported on many fleshy root vegetables such as beets, carrot, radish, and turnip. Rotation with small grains, or alfalfa appears to reduce disease in subsequent potato crops.
Although the mechanism of resistance to scab is not well understood, varieties with different levels of resistance to common scab have been identified through field screening programs. Planting resistant cultivars is probably the best and easiest way to combat common scab. However, resistant varieties are not immune and will become infected if soil inoculum densities are high and conditions are favorable.
Chemical and antimicrobial compounds have been used to control common scab with varying degrees of success. Chemical treatments such as 3,5-dichlorophenoxyacetic acid (3,5-D), benzoic, and picolinic acids tend to cause plant injury. A few antimicrobial compounds have proved effective but none are registered for use in the U.S. The chemical pentachloronitrobenzene (PCNB), also known as Blocker® (Amvac) has been tested. While this has demonstrated some degree of success, studies have indicated that if it is used at higher concentrations (20lbs/A) a decrease in tuber size or yield can result. Pic-plus (chloropicrin) has shown some efficacy in trials in Michigan and Ontario but more so in Florida. The activity of chloropicrin and application requirements such as minimum soil temperatures >45°F and 30-day interval post-application planting restrictions would require fall application in Michigan in most seasons.