We noticed Scab on the radishes at the college farm a couple of weeks ago.
Areas highlighted reference symptoms and controls in particular to
radishes:
Potato
scab is a common tuber disease that occurs throughout the potato growing
regions of the world. Although scab does not usually affect total yields,
significant economic losses result from reduced marketability of the tubers.
Economic losses are greatest when tubers intended for table stock are infected,
since appearance is important for this market. While superficial scab lesions
do not greatly affect the marketability of processing potatoes, deep-pitted
lesions, however, do increase peeling losses and detract from the appearance of
the processed product. The occurrence of scab and its severity varies by season
and from field to field. Cropping history, soil moisture, and soil texture are
largely responsible for this variability. Potato scab lesions can be confused
with powdery scab, a disease caused by an entirely different pathogen, the
fungus Spongospora subterranea (see Cornell Cooperative Extension
Information Bulletin 205: Detection of Potato Tuber Diseases and
Defects).
Symptoms
and Signs
Potato scab
lesions are quite variable and distinctions have been made between russet
(superficial corky tissue), erumpent (a raised corky area), and pitted (a
shallow-to-deep hole) scab All of these can be caused by the same pathogen, Streptomyces
scabies; however, the type of lesion probably is determined by host
resistance, aggressiveness of the pathogen strain, time of infection, and
environmental conditions.
Individual
scab lesions are circular but may coalesce into large scabby areas. Insects may
be involved in creating deep pitted lesions. The term "common scab"
generally refers to the response of the disease to soil pH. Common scab is
controlled or greatly suppressed at soil pH levels of 5.2 or lower. Common scab
is widespread and is caused by S. scabies. "Acid scab"
seems to have a more limited distribution, but has been found in several states
in the Northeast. This disease occurs in soils below pH 5.2, as well as at
higher levels. The causal agent, S. acidiscabies, is closely
related to the common scab pathogen and can grow in soils as low as pH 4.0.
Acid scab is controlled by crop rotation, but can be a problem when seed is produced
in contaminated soils. Acid scab lesions are similar, if not identical, to
those caused by S. scabies.
Disease
Cycle
Most if
not all potato soils have a resident population of S. scabies which
will increase with successive potato or other host crops. Scab-susceptible
potato varieties appear to increase soil populations faster than scab-resistant
varieties. Rotation with grains or other nonhosts eventually reduces but does
not eliminate the S. scabies population. This pathogen is a good
saprophyte and probably reproduces to some extent on organic material in the
soil. Given the right environmental conditions and a scab-susceptible potato
variety, scab can occur in afield that has been out of potatoes for several years.
S.
scabies infects young developing tubers
through the lenticels and occasionally through wounds. Initial infections
result in superficial reddish-brown spots on the surface of tubers. As the
tubers grow, lesions expand, becoming corky and necrotic. The pathogen
sporulates in the lesion, and some of these spores are shed into the soil or
reinfest soil when cull potatoes are left in the field. The pathogen survives
in lesions on tubers in storage, but the disease does not spread or increase in
severity. Inoculum from infected seed tubers can produce disease on progeny
tubers the next season.
The
disease cycle of S. acidiscabies is similar to that of S. scabies,
but the acid scab pathogen does not survive in soil as well as common scab.
Inoculum on seed tubers, even those without visible lesions, seems to be
important in disease outbreaks in New York.
Factors
Influencing Disease Severity
•
Varietal resistance. Though the mechanism of resistance to scab is not
well understood, varieties with different levels of resistance have been
identified through field screening programs. Using resistant varieties is an
effective tool for management of scab. Resistant varieties are not immune,
however, and will become infected given high inoculum densities and favorable
environmental conditions. The limited information available indicates that
there is a good correlation between resistance to common scab and to acid scab
among potato varieties. Consult potato disease-control recommendations for
current information on potato varieties with resistance to scab.
•
Soil acidity. Severity of common scab is significantly reduced in soils
with pH levels of 5.2 and below, but losses can rapidly increase with small
increases in pH above 5.2. Potatoes are commonly grown in soils with a pH of
5.0 to 5.2 for control of common scab. As mentioned, S. acidiscabies
("acid scab") causes scab in low-pH soils. This species does not
compete well with other soilborne microbes, however, and can usually be
controlled with seed treatments and crop rotation.
While
low-pH soils provide good control of common scab, there are disadvantages
associated with this management strategy. Plant nutrients are most available at
soil pH levels near 6.5. Since acid soils are unfavorable for most vegetable
and field crops, the number of them that can be grown in rotation with potatoes
is limited. Maintaining soils near pH 5.0 reduces both fertilizer efficiency
and minor element availability, and may result in phytotoxic levels of some
minor elements. Potatoes grown in soils near pH 6.5 produce higher yields with
less fertilizer. Lack of crop rotation aggravates many pest problems,
especially the Colorado potato beetle.
•
Soil moisture. Soil moisture during tuberization has a dramatic effect
on common scab infection. Maintaining soil at moisture levels above -0.4 bars
(near field capacity) during the 2 to 6 weeks following tuber initiation will
inhibit infection by S. scabies. Bacteria that flourish at high
soil moisture appear to outcompete S. scabies on the tuber surface. However,
maintaining high soil moisture may be difficult in some soils, and it is
possible that other disease problems may be aggravated by excessive irrigation.
•
Soil type and soil amendments. Light-textured soils and those with high
levels of organic matter are favorable to scab infection.Streptomyces are
generally involved in the decomposition of soil organic matter, and therefore
thought to be stimulated by its presence. Applying manure to potato fields can
cause an increase in scab infection. Coarse-textured soils are conducive to
scab, probably because of their moisture-holding capacity; thus, gravelly or
eroded areas of fields that tend to dry out rapidly are often sites of heavy
scab infection.
•
Crop rotation. Crop rotation reduces the inoculum levels in potato
fields, but S. scabies can survive for many years in the absence of potato.
This may be due to saprophytic activity or an ability of S. scabies to
infect other plants. Infection of
seedlings of many vegetables and fleshy roots of beet, cabbage, carrot, radish,
spinach, turnips and other plants has been reported. Rotation with small
grains, corn, or alfalfa appears to reduce disease in subsequent potato crops.
Red clover, however, stimulates problems with common scab and should not be used
in fields where scab has been a problem. S. acidiscabies appears
to have a host range similar to that of S. scabies but does not
survive well in the presence of nonhost crops.
Recommended
Disease-control Strategies
1. Use resistant
varieties in fields where scab is a problem
2. Use scab-free seed and seed treatments to prevent
introduction of the pathogen into fields. Seed treatments do not eliminate the
pathogen but will provide some suppression of disease. Consult current potato
disease-control recommendations for appropriate seed treatments.
3. Rotate heavily infested fields away from
potatoes and alternate hosts such as radish, beets, and carrots. Use small
grains, corn, or alfalfa in rotations; avoid red clover.
4.
Maintain soil pH levels between 5.0 and 5.2 by using acid-producing fertilizers
such as ammonium sulphate. Avoid or limit the use of such alkaline-producing
amendments as lime and manure.
5. Avoid moisture stress during the 2 to 6
weeks following tuberization.