Common Scab: The Silent Invader Beneath the Soil Surface

Introduction: The Hidden Menace Beneath the Soil

Common scab is one of the most widespread soil-borne diseases of potato (Solanum tuberosum). It is caused by plant-pathogenic bacteria in the genus Streptomyces, mainly Streptomyces scabiei (long written S. scabies), together with S. acidiscabies, S. turgidiscabies, S. europaeiscabiei, S. stelliscabiei and a growing list of related species. The disease forms rough, corky lesions on the tuber skin, from shallow russet patches to raised scabs and deep pits. It rarely reduces total yield, and scabby potatoes are safe to eat, but the blemishes cut the value of fresh, seed and processing crops and can lead to whole loads being downgraded or rejected.

At the centre of the disease are toxins called thaxtomins. Thaxtomin A blocks cellulose synthesis in young tuber cells, and the plant seals off the dying tissue with cork. Infection takes place through the lenticels of young, expanding tubers, so the weeks just after tuber initiation decide how much scab a crop carries at harvest. Severity rises as soil pH increases from 5.2 to 8.0 and is worst above pH 7.0, while warm, dry soil during tuber set sharply raises the risk, especially on sandy and gravelly land.

Common scab occurs wherever potatoes are grown. A Canadian grower survey put annual losses there at C$15.3–17.3 million in 2002, and buyers commonly reject tubers once lesions cover more than 5% of the surface. Because no single treatment gives reliable control, growers combine resistant potato varieties, clean seed, well-timed irrigation, soil acidity management and rotation with non-host crops. Current research looks at the tuber-surface microbiome and at plant-based soil amendments, including Ginkgo biloba leaf litter, as ways to make soils naturally suppressive to the pathogen.

Potato Common Scab: Key Facts and Figures

Potato tuber exhibiting typical corky lesions from common scab infection

History: Tracing the Roots of Common Scab

Scabby potatoes were familiar to farmers long before anyone knew what caused them. In 1890 Roland Thaxter, working in Connecticut in the United States, isolated a filamentous organism from scab lesions and published it as Oospora scabies in the early 1890s. It was among the first plant diseases traced to an actinobacterium, a group of bacteria that grow as branching threads rather than single cells, which is why the cause was mistaken for a fungus at first. The history of the potato in Europe and North America is closely tied to these early disease investigations.

The name changed as the organism was better understood. Güssow moved it to Actinomyces scabies in 1914, and Waksman and Henrici placed it in Streptomyces in 1948. Thaxter's original culture had been lost, so Lambert and Loria re-described Streptomyces scabies from modern strains in 1989. Bacterial naming rules later corrected the spelling to S. scabiei, a decision confirmed in 2020, which is why both spellings still appear in the literature and on seed certificates. Field studies through the early twentieth century, in parallel, established that soil pH and soil moisture during tuber formation govern how much scab develops. Those two findings still underpin cultural control today.

From the late 1980s the picture widened considerably. Lambert and Loria also described S. acidiscabies in 1989, from potato fields in Maine, showing that a second species could cause so-called acid scab in soils below pH 5.2 where S. scabiei is held in check. In the same year researchers isolated the thaxtomins and identified thaxtomin A as the compound responsible for the lesions. S. turgidiscabies was described from Japan in 1998 and was subsequently reported in Finland, Sweden and the United Kingdom. The sequencing of the S. scabiei genome, about 10.1 million base pairs and one of the largest known in the genus, and the discovery of a transferable pathogenicity island then explained why new scab-causing species keep turning up in surveys.

Global Distribution and Spread of Common Scab

Common scab occurs in every region where potatoes are grown. It is a recurring problem in major producing countries including Canada, the United States, China, India and Australia, and it is most damaging on light, free-draining soils with a pH above 5.2 and in seasons when tuber set coincides with dry weather.

Which species dominates varies from region to region. S. scabiei is the most widespread and is the main pathogen across temperate North America and Europe, where S. europaeiscabiei and S. stelliscabiei are also common; a national survey in Germany found all three, with their distribution shaped by soil type and pH. S. acidiscabies is associated with acid soils, mainly in the north-eastern United States, Japan and Korea. S. turgidiscabies has been reported from Japan, Finland, Sweden and the United Kingdom. Netted scab, a finer surface symptom, is attributed to S. reticuliscabiei and to some strains of S. europaeiscabiei, and is now treated as a separate disease.

New reports continue to extend the list. S. rhizophilus was shown to cause common scab in Heilongjiang, China, in 2022, where it was found in about 2% of scabby samples tested, and S. niveiscabiei, first described in Korea, has since been identified on scabby tubers in Shanxi and Gansu provinces. Surveys in Türkiye and South Africa have likewise found mixtures of several species in the same production areas, which matters for management because the species differ in the soil pH they tolerate.

The pathogen moves between fields mainly on infected seed tubers and in soil carried by machinery, boxes, boots and harvest equipment. Wind-blown soil, surface run-off and irrigation water spread it locally, and manure from animals fed scabby potatoes can introduce it because the spores survive digestion. Once a field is infested, Streptomyces persists for years as a saprophyte on organic matter, roots and crop residues, so rotation reduces inoculum but does not clear a field. Because the pathogen is soil-borne and already widespread, quarantine has little role; scabby seed remains the main route by which the disease reaches clean land, which is why seed certification schemes set tolerances for scab.

Host Range and Cross Infection Dynamics of Common Scab Pathogens

Potato is by far the most important host, but S. scabiei also causes scab on beet, carrot, parsnip, radish, rutabaga and turnip. These hosts sit mainly in the Solanaceae, Apiaceae, Brassicaceae and Amaranthaceae (formerly Chenopodiaceae) families, and pathogenic Streptomyces strains also cause pod scab of peanut in some regions. Sweet potato is affected by a different species, Streptomyces ipomoeae, which causes soil rot or pox rather than common scab, so sweet potato should not be listed as a common scab host.

Cross-inoculation studies show that isolates from one host can often infect another, so isolates from carrot or radish may produce lesions on potato, though virulence differs between strains and lesion type can range from superficial russeting to deep pitting. That overlap is the reason beet, carrot, parsnip and radish are best avoided in rotations on land with a scab history. Cereals and grasses do not develop scab, so small grains, maize, alfalfa and grass leys are the usual rotation choices, although rotation reduces the pathogen without eliminating it. Red clover has a long-standing reputation for making scab worse and is generally avoided on infested fields.

Most members of the genus are harmless soil bacteria, and some non-pathogenic Streptomyces strains isolated from scab lesions or from naturally suppressive soils compete with the pathogen or produce antibiotics against it. Several are being developed as biological seed and soil treatments. Volunteer potatoes and solanaceous weeds such as nightshade allow the pathogen to keep multiplying between potato crops, so controlling them is part of effective rotation and part of general crop protection practice.

Global Economic Burden and Quality Losses Due to Common Scab

Common scab costs money through quality, not quantity. Plants look healthy, total tuber weight is usually little changed, and the tubers remain perfectly edible, but scabbed potatoes are graded down, sold into lower-value outlets or rejected outright. For processors, deep lesions have to be peeled away, which raises peeling losses and lowers recovery. For fresh packers, appearance decides the sale. In seed crops, severe scab can damage the eyes and push a lot into a lower class.

The best-documented estimate comes from Canada, where a grower survey put losses from rejected and downgraded tubers at C$15.3–17.3 million for 2002, with the largest provincial total in Prince Edward Island, which has the most potato growers. There is no equivalent national loss figure for the United States, where the disease is instead tracked through grading and field surveys, and the same is true of most other producing countries. Globally the cost is widely described as running into the billions of dollars, but that is an industry estimate rather than a measured total.

What makes the disease expensive is the low tolerance of buyers. Tubers with lesions covering more than 5% of the surface are generally rejected by the industry, so a modest level of infection can make a consignment unsaleable in the fresh market. Losses are therefore concentrated in washed and pre-packed lines, export consignments and thin-skinned salad and baby potatoes, where every blemish is visible. Crisping and french fry crops tolerate light surface scab better, but deep pitted scab still cuts peeling recovery.

Control carries its own cost. Irrigation through tuber set, sulphur or other acidifying amendments, seed treatments, longer rotations and, where permitted, soil fumigation all add to production costs, and irrigation is becoming more expensive where water is scarce. Because none of these measures is reliable on its own, growers in high-risk fields often pay for several at once, which is the practical reason resistance breeding attracts so much research funding.

Potato tubers with common scab lesions, illustrating economic impact on marketability.

Biology, Life Cycle and Pathogenicity of Streptomyces scabies

Streptomyces scabiei is a Gram-positive, filamentous soil bacterium. Like other streptomycetes it grows as a branching network of hyphae and reproduces by forming chains of spores on aerial hyphae, which is why its colonies look powdery and why its spores survive drying so well.

Survival between crops: The pathogen lives in soil as a saprophyte on organic matter, roots and crop residues, and can persist for years in the absence of a potato crop. Because it feeds on organic matter, fresh manure and some organic amendments can increase its activity. Infested soil and infected seed tubers both carry it forward, which makes common scab both soil-borne and seed-borne, not soil-borne alone.

Infection: Spores germinate near developing tubers and the hyphae enter through young lenticels, and possibly directly through the immature skin. Tubers become susceptible as soon as they form, and remain so while the skin is still expanding; a mature tuber with good skin set is no longer susceptible to new infection. Stems, stolons and roots can also carry shallow lesions. This narrow window is why control measures are timed to tuber initiation rather than to planting or harvest.

Toxin production: Pathogenic strains carry a pathogenicity island holding the thaxtomin biosynthesis genes txtA, txtB and txtC with their regulator txtR. Cellobiose, a sugar released from expanding plant cell walls, binds TxtR and switches toxin production on, which is how the bacterium senses that a young tuber is nearby. Thaxtomin A then inhibits cellulose synthesis, the affected cells swell and die, and the tuber walls off the damage with cork, producing the familiar lesion. Other factors on the island, such as the secreted protein Nec1 and the enzyme TomA, are not essential for disease but contribute to more severe raised and pitted lesions. Reports that thaxtomin is controlled by quorum sensing are not supported; the trigger is the plant-derived sugar signal.

Why new species keep appearing: The pathogenicity island can be mobilised and transferred to non-pathogenic relatives, so a harmless soil streptomycete that acquires it can become a scab pathogen. That mechanism explains the steady stream of newly described scab-causing species, and both S. acidiscabies and S. turgidiscabies carry versions of the same island. S. acidiscabies is important in practice because it tolerates acid soils and can infect tubers below pH 5.2, where acidification alone no longer protects the crop.

Microscopic image of Streptomyces scabies hyphae and spores.

Factors Influencing Disease Severity in Common Scab

Varietal resistance: Varieties differ widely in how much scab they carry under the same conditions, although the mechanisms are still only partly understood; thicker skin and fewer, tougher lenticels are associated with better resistance. No variety is immune, and high inoculum with favourable weather will produce scab even on resistant types. Among North American varieties, Superior, Nooksack, Dark Red Norland and Russet Burbank carry some resistance, and Pike, Keuka Gold, Lehigh, Marcy and Dakota Pearl are also used where scab is a known problem. Yukon Gold, Kennebec, Katahdin, Shepody and Russet Norkotah are among the more susceptible, and russeted varieties generally show scab less than smooth-skinned ones. In Australia, early types such as Red Pontiac, Kennebec and Sequoia are more susceptible than later-maturing Sebago and Coliban. Resistance to common scab does not always carry over to acid scab, so local trial data matters more than a general rating.

Soil acidity: Soil pH is the single strongest factor. Severity increases from pH 5.2 upwards and is worst in neutral to alkaline soils, which is why holding soil at pH 5.0 to 5.2 is a long-standing control practice. The approach has limits: results from acidification are inconsistent and impractical in many fields, S. acidiscabies can still infect below pH 5, most nutrients are most available nearer pH 6.5, and strongly acid soils restrict which crops can follow in the rotation and can cause trace-element problems.

Soil moisture: Moisture during tuber set strongly influences infection, because a moist soil supports the competing microbes that hold the pathogen back. Keeping the ridge near field capacity from tuber initiation onwards suppresses infection, while dry spells in that window favour it. The critical control period runs from one to three weeks after tuber initiation, with irrigation continued for roughly three to four weeks in maincrops and up to about six weeks in susceptible salad varieties, keeping the top 25 cm moist. Over-watering in the first week, however, restricts rooting and encourages other diseases.

Soil type and amendments: Light, coarse-textured soils that dry quickly carry the heaviest scab, and gravelly or eroded patches within a field are often the worst affected. Because Streptomyces species are decomposers, high organic matter and fresh manure applications can increase disease; manure may also raise soil pH and, if the animals were fed scabby potatoes, introduce the pathogen directly. Gypsum is preferred to lime where calcium is needed, since lime raises pH.

Crop rotation: Rotation lowers inoculum but never removes it, because the pathogen survives saprophytically and on alternate hosts including beet, carrot, radish, turnip, spinach and cabbage seedlings. Three to five year rotations with cereals, maize, alfalfa or grass give the most consistent benefit. Red clover should be avoided on infested land, and volunteer potatoes and solanaceous weeds need controlling so the rotation is not undermined.

Symptoms and Damage of Common Scab

Common scab produces three broad symptom types on the tuber skin, and all three can appear in the same crop. Surface or russet scab shows as tan to brown, roughened corky patches that stay level with the skin. Raised or erumpent scab forms cushions of corky tissue standing above the surface. Pitted scab forms roughly circular lesions up to about 10 mm across and 1 to 10 mm deep, surrounded by corky tissue, and is the most damaging form because the pits must be cut or peeled out. Lesions are brown to almost black, often circular but sometimes merging into irregular patches that cover much of the tuber.

The first infections appear as small tan to reddish-brown spots on young tubers and enlarge as the tuber grows, so the amount of surface affected at harvest depends on how early infection started. Netted scab, now attributed to S. reticuliscabiei and some S. europaeiscabiei strains, gives a finer, net-like surface roughening rather than discrete lesions. Deep pitting is more common in dry soils, while shallow russeting predominates in other conditions, and symptom type also depends on which species and strain is present, since strains carrying nec1 and tomA tend to cause more severe raised and pitted lesions.

Foliage shows nothing. There is no wilting, yellowing or stunting, so infection cannot be judged from the above-ground crop and the first sign is usually at harvest or grading. Shallow lesions may also occur on stems, stolons and roots. Similar scabbing appears on carrot, beet, radish and turnip where those crops are grown on infested land.

The damage is cosmetic but commercially serious. Peeling losses rise, graded-out percentages rise, and washed or pre-packed lines suffer most because lesions are obvious on a clean skin. Scabbed tubers are safe to eat and store normally, although deep pits that are wounded at harvest can let soft-rot bacteria in, so careful handling matters in badly affected crops. In seed potatoes, scab on and around the eyes is a grading defect in its own right.

Symptom and Sign of Scab

Management Strategies for Common Scab

There is no single reliable treatment for common scab, so control rests on integrated management: a resistant variety, clean seed, the right water and pH, a long enough rotation, and chemical or biological treatments only as a supplement. The measures below work best in combination, and the gains from each one vary between fields and seasons.

Resistant varieties: Variety choice is the most dependable tool and the cheapest to apply. Differences between varieties are consistent and large enough to decide whether a field can be cropped at all, which is why genetic resistance is described as the most reliable and cost-effective strategy. Match the variety to the field: keep susceptible salad and pre-pack varieties off land with a scab history, and use the most resistant types available where scab is unavoidable.

Clean seed: Planting certified seed free of visible lesions is the main way to keep the pathogen out of clean fields. On already-infested land its benefit is smaller, since seed with light and heavy scab produced similar scab on daughter tubers in trials, which shows how much of the inoculum is already in the soil.

Irrigation through tuber set: Keeping the ridge near field capacity from tuber initiation is the main in-season control. Irrigate from the start of tuber initiation and continue through the critical period, applying water evenly and checking that it reaches the tuber zone. Stopping too early allows the pathogen to multiply again, and the practice works less well in soils that are hard to wet evenly.

Soil acidity: Acidifying to pH 5.0 to 5.2 with elemental sulphur or acid-forming fertilisers such as ammonium sulphate reduces severity in neutral and alkaline soils, and dolomitic lime, if needed at all, is best applied after the potato crop in the rotation. Results are inconsistent, sandy soils may need repeat applications, and the approach does not work against acid scab.

Rotation and green manures: Three to five year rotations with non-hosts such as cereals, maize or grass reduce inoculum. Brassica and other green manures, including buckwheat, canola, oat, rye and millet, are reported to suppress Streptomyces, and mustard green manures are the most widely tested, though field results vary.

Chemical options: No product gives dependable control, and availability differs sharply by country. PCNB (quintozene) is registered in the United States but not in Canada, and its performance has been patchy. A fludioxonil seed treatment reduced scab incidence by about 35% and raised marketable yield by about 25% in a Canadian study. Chloropicrin row fumigation has given 60 to 80% reduction in Ontario trials but is costly and unsuitable after manure or maize. Foliar auxins such as 2,4-D have reduced scab in some studies, probably by inducing plant defences. Any treatment supplements resistance and clean seed rather than replacing them.

Biological control and the soil microbiome: Non-pathogenic Streptomyces strains, Bacillus and Pseudomonas isolates and compost amendments are all being developed to make soils more suppressive. Results so far are promising but inconsistent across sites and seasons, and this is the most active area of current research.

Prevention and Good Practices for Common Scab

Prevention is cheaper than treatment, because once a field is infested the pathogen stays. The aim is to keep clean fields clean, and to make infested fields less favourable for infection during the few weeks that matter. Good soil and seedbed management does most of the work.

Start with clean seed and clean ground: Use certified seed with no visible lesions, and avoid planting susceptible varieties on land that has grown scabby crops. Keep records of which fields produced scab and at what severity, since field history is the best available predictor of risk.

Know the field before planting: Test soil pH and plan amendments well ahead, as elemental sulphur takes months to act. Where soils are naturally above pH 7.0, acidification will be slow and incomplete, and variety choice and irrigation will have to carry more of the load. Laboratory qPCR tests for pathogenic Streptomyces and the txtAB genes exist and are used in research and in some advisory services, but they are not yet routine in commercial practice.

Plan water before tuber initiation: Decide in advance how the crop will be watered through tuber set, and check that the irrigation system is working before tuber initiation rather than after. Even application matters more than total volume, and repeat applications every few days keep the ridge at field capacity. Uneven emergence stretches the susceptible period across the field, so good seed handling, even planting depth and uniform cultivation all shorten the window that has to be protected.

Manage organic inputs: Avoid fresh animal manure before potatoes, both because it feeds the pathogen and because manure from animals fed scabby potatoes can carry viable spores. Compost well, apply earlier in the rotation, and avoid wood ash and excess lime, which raise pH.

Keep soil from moving: Clean soil off machinery, boxes, trailers and boots between fields, and be careful with harvest equipment moving from infested to clean land. Avoid spreading infested soil through grading waste or washings, and control volunteer potatoes and weed hosts such as nightshade so the pathogen has nothing to multiply on between crops.

Handle the harvest well: Allow skins to set before lifting. A mature skin is no longer susceptible, and careful handling reduces wounding of pitted lesions, which is what lets secondary rots in during storage.

Future Threats to Potato Production from Common Scab

Common scab is not a new or emerging disease, but several trends are making it harder to manage. The threats below are directional rather than quantified: for most of them there is no reliable published figure for how much worse the disease will become, and the honest position is that the direction is clear while the size is not.

Warmer, drier soils at tuber set: The disease is favoured by warm, dry soil during tuber initiation, with daytime temperatures above about 21°C and low soil moisture through that period. Where climate change brings hotter, drier spells at tuber set, or makes irrigation water less reliable, risk rises on exactly the light soils that already carry the most scab. Erratic rainfall is a particular problem because moisture-based control depends on continuity, not on seasonal totals.

New species and strains: The pathogenicity island moves between streptomycetes, so new scab-causing species continue to be described, including S. rhizophilus and S. niveiscabiei in China in recent years. Each new species may differ in the soil pH, temperature and host range it tolerates. Acid-tolerant species are the main concern for management, because acidification, one of the few cultural levers available, does not work against them.

Fewer chemical options: Regulatory review continues to narrow the list of permitted soil treatments and fumigants, especially in the European Union, and no new product has replaced them. That shifts the burden onto resistance, water management and biological approaches, none of which is yet consistent enough to stand alone.

Interaction with other problems: Anything that wounds tubers or stresses the crop makes scab worse in practice. Wireworm and other soil pest damage creates entry points and adds to grading losses, and crops already under drought or nutrient stress have less capacity to form a protective skin quickly. Where these problems coincide, the graded-out percentage rises faster than scab alone would suggest.

Uneven access to the tools that work: The measures that control scab best, resistant varieties, certified seed, soil testing and irrigation through tuber set, all need capital, water or an advisory service. Growers without reliable access to them, particularly smallholders in emerging potato-producing regions, carry more of the loss. Extending seed systems and variety testing into those regions is therefore as much a part of scab management as any new technology.

Management Challenges in Controlling Common Scab

Common scab is difficult to control for reasons that are built into the pathogen and the crop, not simply a matter of better products. Understanding those limits helps explain why the disease persists on well-managed farms.

The pathogen cannot be removed from a field: Streptomyces species survive as saprophytes on organic matter and roots and produce drought-tolerant spores, so eradication is not a realistic goal and rotation alone will not clear infested land. Management aims at keeping infection below the grading threshold rather than at eliminating the pathogen.

Risk is hard to measure before planting: Low populations can cause damaging disease, pathogenic and harmless streptomycetes live side by side in the same soil, and soil assays are not well correlated with what the crop will show. qPCR for the txtAB genes is more specific and is used in research, but cost and interpretation keep it out of routine commercial use, so field history remains the main guide.

No reliable chemical control exists: This is the central practical problem. PCNB performance has been inconsistent, fumigants are expensive, restricted in many countries and lose effect if the soil is disturbed afterwards, and seed treatments help but do not address soil inoculum. As an older review of the Canadian situation put it, lowering pH below 5.2 is inconsistent and impractical for many fields, and the same caution applies to most of the chemical options.

Cultural practices vary in effect: Irrigation through tuber set and soil acidification both work, but their effect depends on soil texture, water availability and how evenly the crop emerges. Light soils need frequent, even watering to stay near field capacity, and sandy soils may not hold an acidifying amendment for long. Long rotations are effective but compete with land availability and cash flow, which is why they are often shortened in practice.

Resistance is partial and slow to breed: No commercial variety is fully resistant, and the mechanisms are not well enough understood to select for directly. Resistance appears to be quantitative, involving several loci plus skin and lenticel characteristics, and molecular markers are limited, which slows breeding. One promising route has come from tissue culture: a somaclone of Russet Burbank selected for tolerance to thaxtomin A, RB9, produced up to 22% more marketable tubers than the parent variety, with resistance holding for at least two field seasons.

Biological control is not yet dependable: Suppressive soils clearly exist, and specific bacteria are associated with lower severity, but moving from that observation to a product that performs across soils and seasons has proved difficult. Temperature, soil chemistry and the resident microbial community all affect whether an introduced strain establishes.

New Research in Common Scab Management

Most of the recent progress on common scab has come from two directions: understanding why some soils suppress the disease, and finding ways to shift the soil community in that direction without relying on chemicals.

The tuber-surface microbiome: Attention has moved from the bulk soil to the thin layer of soil clinging to the tuber itself, sometimes called the geocaulosphere. Work in Shandong, China, found that scab severity, the abundance of pathogenic Streptomyces and txtAB gene copy numbers all correlated with the composition of that tuber-surface community, with Variovorax, Stenotrophomonas and Agrobacterium more abundant where scab was severe, and Bacillus, Geobacillus and Curtobacterium associated with low severity. The study was correlational, so it points to candidates for biological control rather than proving cause, but it reframed scab as an outcome of community balance rather than of pathogen presence alone.

Plant-based soil amendments: The same line of work has led to trials of agricultural and forestry residues as suppressive amendments. A 2025 study, using Ginkgo biloba leaf litter against potato common scab, reported suppression linked to shifts in the soil microbial community. Approaches of this kind are at the field-trial stage, and their practicality depends on how much material is needed per hectare and whether it is available locally, so they are best read as a promising line of research rather than a recommendation.

Antagonistic Streptomyces strains: Non-pathogenic streptomycetes isolated from scab lesions and from suppressive soils are being tested as seed and in-furrow treatments, and trials in Finland have shown that such strains can reduce scab and change the bacterial community around the tuber. The challenge is consistency, since establishment depends on soil conditions and the resident community.

Breeding and selection for resistance: Resistance is quantitative, so breeders are mapping loci and looking for markers that would speed up selection. A different route has used the toxin itself: potato tissue gradually habituated to thaxtomin A produced a Russet Burbank somaclone with better field resistance and a thicker skin layer, an approach that works without genetic modification. Alongside this, public breeding programmes are screening germplasm for scab resistance genes to bring into commercial varieties.

Diagnostics: qPCR assays targeting the thaxtomin biosynthesis genes can quantify pathogenic Streptomyces in soil and distinguish them from the harmless majority, which the older culture-based assays could not. They are used in research and in some advisory programmes; bringing cost and interpretation to the point where growers can make planting decisions from a soil test is the remaining step.