Note: These notes have been put together by Dr. Jeff Miller, potato
pathologist at the University of Idaho (208-397-4181). These notes were taken by Dr. Miller and he is
responsible for the interpretation of what was presented at the Powdery Scab workshop. The workshop was
sponsored by Colorado Certified Seed Potato Growers Association and Colorado State University Cooperative
Extension. Rick Zink and Rob Davidson were the workshop organizers.
- Meeting presenters:
- Dr. Stuart Wale, Head of Crop Services, Scottish Agricultural College
- Dr. Barbara Christ, Professor of Plant Pathology, Penn State University
- Dr. Melvin Grove, Business Development Manager, ISK Biosciences
- Biology of Spongospora subterranea f. sp. subterranea (by Dr. Wale)
The causal agent of powdery scab is Spongospora subterranea f. sp. subterranea. This organism
produces naked plasmodia. It is considered a water mold. The organism is an obligate parasite, but is very well
adapted. Zoospores need water to swim and can only travel very short distances (a few millimeters at the most).
The life cycle takes about 10-14 days. However, the time from zoospore release (from a spore ball) to root
infection is one hour. Spore balls are relatively large (20-100 mm in diameter) and contain hundreds of cysts.
These spore balls are resistant to many environmental conditions. Swimming spores can be released from these
spore balls for up to 20 years.
Swimming spores are very short lived and die in dry soils. When released, they can infect root hairs, stolons, and
tubers. Secondary zoospores can be seen prior to release in root hairs with the aid of a microscope. Once a
zoospore has infected plant tissue, host cell enlargement and growth are stimulated. Infection of tubers can
cause unsuberized or proliferating lenticels.
When symptoms are seen all over the tuber, infection occurred at tuber initiation. If symptoms appear in a zone
on the tuber, infection occurred after tuber initiation. First symptoms visible on tubers include small bumps which
look like pimples. These turn into raised, white pustules, which become brown. The periderm then bursts
producing dark, erumpent pustules. Tuber lesions may not be visible until harvest or later. When tuber eyes
become infected, severe tissue swelling can occur along with malformation of the tubers.
The Spongospora "fungus" can vector the mop-top virus. Symptoms of mop-top virus in potato tubers include
brown, slightly raised lesions. Internal necrosis can be ½ inch deep.
- Disease Symptoms (by Dr. Christ)
Symptoms on tubers are typically raised pustules with a powdery mass. Spore balls, which are found in these
lesions, can be seen with the aid of a microscope. Seeing the spore balls is the one sure way to identify the
presence of powdery scab. Root galls will increase throughout the season. It a tuber has "pimples," set is aside
for 24 hours. The color will change if the lesion is powdery scab.
Symptoms can develop after harvest. Tuber to tuber spread in storage has never been documented.
Contamination of seed pieces with spore balls is possible, however. Infected tubers may shrink in storage.
Early lesions are small and purple. These are easy to pass over as being something other than powdery scab.
If you are in doubt, set tubers with these small purple lesions aside and wait for a day or two. Spore balls will
form if it is indeed powdery scab.
Stem infections occur near the soil line. Tubers with deep pitted lesions seem to have fewer spore balls.
Russet varieties will not show many symptoms, but can perpetuate the disease. Other varieties will show
symptoms more dramatically. To see powdery scab, look in wet areas first.
Rhizoctonia, and common scab symptoms can also look like powdery scab.
- Distribution of Powdery Scab (by Dr. Wale)
Geographic distribution of the disease includes Canada, United States, South America (origin of the pathogen),
New Zealand, Australia, Pakistan, Israel, South Africa, India, Northern Europe, the United Kingdom, and Ireland.
Some soils appear to suppress powdery scab. The reasons for this suppressiveness is not well understood..
Inoculum can come from infected or contaminated seed tubers, soil contamination, contaminated manure or slurry
spread on a field, field to field movement of soil (infested equipment, boots, etc.), and perhaps as wind-borne
Potato, tomato, and solanaceous weeds such as nightshade are hosts. Volunteer potatoes may also be
important as a host. Root infections have been observed on wheat, barley, oats, perennial ryegrass, oilseed
rape, turnips, chickweed, and clover. However, these are dead end infections because the pathogen cannot
multiply on these crops.
Factors which affect powdery scab include:
- Soil moisture
- Soil type
- Cultivar resistance
- Crop rotation
- Soil moisture
Free water in the soil matrix is needed for zoospore movement. Soil temperatures of 11-18°C (52-64°F) favor the disease. Alternating periods of wet and dry soil also favor the disease. If conditions are favorable prior to tuber initiation, infections will occur at the stolon end. If conditions are favorable at tuber initiation, infections will occur over the entire tuber. If conditions are favorable after tuber initiation, infections will occur at the bud end.
High soil moisture (above -0.3 bar) is required for infection. Soil moisture below this results in little infection. Low O2 also favors the disease. The tuber growth slows and the window of time for infection increases.
Suberin appears to confer resistance. The more tubers mature, the more resistant they appear.
Infection occurs around 11-15°C (52-59°F), which often coincides with tuber initiation. Tuber initiation is the most susceptible time for infection (3-4 weeks after first hook). The bud end is susceptible during tuber swelling.
- Soil type and pH
Soils with a high moisture holding capacity have more disease. Light soils can also be favorable where wet and dry cycles occur. The effect of pH is very small. As pH increases, disease can be worse.
- Cultivars (by Dr. Christ)
Screening of cultivar resistance to powdery scab is done at Penn State University. It has taken a few years to get a good handle on cultivar rankings. Planting later has reduced the incidence of powdery scab. This is ascribed to an increase in soil temperatures (above the favorable range) during the susceptible period. To measure powdery scab, rating incidence (# of tubers infected) seems the best method. Disease severity is much more difficult to score, and the cultivar rankings are similar as when incidence is used.
The designation of resistance and susceptibility is somewhat arbitrary. Kennebec is considered one of the most
susceptible cultivars. Susceptibility rankings for tuber symptoms are as follows:
- Highly Susceptible
- Red Norland
- Dark Red Norland
- Moderately Susceptible
- Russet Burbank
These ratings do NOT correlate with root galling. Smooth skinned varieties appear to be more susceptible. However, there are some exceptions to this.
Adding inoculum to the soil did not make disease worse in the Penn State plots. Cultivar resistance and duration of favorable soil conditions are more important. When conditions are suitable, low levels of inoculum can build up. A small amount of fungus can result in a large amount of disease. On the other hand, when the environment is unfavorable, disease escape is possible. This means that even though the cultivar is susceptible and the inoculum is present, disease does not occur.
- Control of Powdery Scab (by Dr. Christ)
Avoidance - planting in fields which do not have the powdery scab organism - is the best was to control the disease. However, this is often not practical. Some cultural practices create conditions which are less favorable for disease, such as planting later. Control strategies which rely on reducing inoculum (either tuber or soil-borne) have not worked very well. Other possibilities include the use of resistant cultivars, chemical control, biological control, risk assessment and integrated control, and legislation.
Virgin fields or fields with very low inoculum levels will allow for avoidance of disease. There are bioassays and molecular techniques for detecting the number of spore balls in the soil. One current bioassay can detect as little as 0.3 spore balls/gram soil. A molecular technique based on the polymerase chain reaction (PCR) is not quite as sensitive, detecting as little as 1 spore ball/gram soil. The problem with these assays (as pointed out previously) is that the relationship between inoculum and disease is not always strong. However, a knowledge of soil inoculum levels may be beneficial in deciding which cultivar to plant.
Cultural practices which reduce powdery scab include avoiding soil compaction, providing even irrigation, creating a well-structured seed bed, and avoiding the over-cultivation of soil and creating or planting in soil with too fine of soil tilth. Planting in fields with good drainage and using the appropriate tillage for the given soil type will also reduce disease. The spread of inoculum can be reduced by cleaning grading machines between lots and maintaining good hygiene in storage. Peroxyacetic acid (Oxidate) and chlorine based products have been used for this, but their efficacy is unknown. Additionally, chlorine based products quickly lose efficacy with soil contact. Grading out infected tubers is time consuming and still leaves clean tubers which have become contaminated with spore balls. Hot water dipping of tubers at 55°C (131°F) has not worked in reducing disease. Composting infected material may kill the pathogen. A few spore balls may survive, however. The temperatures during composting must be high.
It is unknown what to do when a field of potatoes has a severe outbreak of powdery scab. Do you harvest and remove the tubers from the soil? Or do you leave them to rot? It may not really matter.
Reducing inoculum can be done by avoiding the use of contaminated manure. Spore balls survive passage through cattle which are feeding on infected potatoes. Also, the elimination of alternative hosts, namely volunteer potatoes, will be helpful. Long rotations will also reduce the amount of inoculum.
Chemical control has been shown by using zinc, fluazinam (Omega), mancozeb, and metam sodium. Sulfur has NOT shown any control. Soil with high levels of zinc have been suppressive to powdery scab. The addition of zinc oxide reduces severity, but not incidence. The rate for tubers is 1 kg/ton (roughly equal to 0.1 lb/cwt) and the rate in soil is 15 kg/ha (about 13 lbs/acre). Availability of Zn is more important than the amount, however. In alkaline soils, Zn can be tied up.
Fluazinam has shown some efficacy in reducing powdery scab. Phytotoxicity has been observed with tuber application. Soil incorporation is probably the best way to apply. Split applications (3X), incorporated into beds gave the most healthy tubers. The rate which has shown efficacy is 3 kg/ha (about 2.6 lbs/acre).
Metam sodium has showed limited efficacy at 750 and 1200 L/ha (79 and 127 gal/acre). Inoculum is not eliminated, however. These rates are probably not cost effective.
Control with mancozeb has been achieved at 8 lbs/acre. However, this is well above label.
Biological control has been seen in soils with high zinc content. Trap crops have been used, but the field is taken out of production for a year. Green manures may be effective when the green manure crop has a high glucosinolate content. Crops should be planted in the fall, chopped in the spring, and incorporated.
An integrated approach to control would involve planting later, modifying irrigation and the use of chemical control.
- Update on Fluazinam (by Dr. Grove)
Fluazinam is registered in Japan, New Zealand, Europe, Brazil, Chile, Columbia, and Mexico. It is very safe on crops and has good disease control efficacy. A section 3 label is expected for potatoes in 2001. It was granted reduced risk by EPA on April 27, 1999. The compound is not very mobile in soil and degrade rapidly. It is non-toxic to bees and other beneficial insects. Since the compound is a multi-site inhibitor of fungal enzymes, there are no resistance concerns and it is a good product for resistance management strategies.
The labeled rate for late blight will be 5.5 fl. oz./A. The label for white mold will be 5.5-13 fl. oz./A. Under "normal" white mold pressure, 8 oz is needed and the full 13 oz should be used in conditions of high pressure. There is a limit of 3.65 pints/A/year and a 14 day PHI. Applications can be made on a 7-10 day interval. The cost is estimated at $320 per gallon.
Due to the limited mobility, the chemical needs to be placed where it will come in contact with the pathogen.
- Colorado experience with powdery scab control (by Dr. Zink)
Fluazinam at 7 pt/A worked well. (This is well above the labeled rate.)
Quadris and Blocker in furrow did not work well.
- In-furrow = 0.7% disease
- 1/3 preplant + 1/3 over seed + 1/3 at closing = 0 disease
- On top of closed row = 0 disease
- Pennsylvania experience with powdery scab control (by Dr. Christ)
Fluazinam was just as effective at the 3 pt/A rate as at the 7 pt/A rate. Additionally, Evolve (Tops-MZ-CZ) also reduced powdery scab.
- Final thought from Question/Answer session
Tubers which set later may appear resistant (disease escape).
There MAY be an interaction between powdery scab and pink rot.