Thanks for the question.
"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)...
"There are actually two types of scab: powdery scab and common scab.
Common scab is just that, more commonly encountered in Kern County
potato fields, while powdery scab is much less of a problem. Although
the symptoms can sometimes be very similar, they are caused by two
different organisms and require different treatments.
Management of common scab requires several different strategies. The
problem can be avoided by not planting scab-infected seed. seed
treatments can reduce the amount of infection if the scab is
seedborne. Saturated soils inhibit common scab. Keeping moisture
levels in the field at or near field capacity as much as possible
during the period that tubers are developing (four to six weeks) helps
Soil pH also can greatly influence the development of common scab. A
soil pH of 7 is ideal for scab development. Fields with a history of
common scab should maintain a pH of 5.5 or slightly lower.
Dealing With Powdery Scab
Powdery scab can be confused with the more severe forms of common
scab. It can become a chronic problem once it becomes established in a
field...the use of clean seed is very important. Make sure the seed
comes from an area that does not have powdery scab. Powdery scab is
aggravated by wet soils which can cause more severe lesions. If it
does become established in a field, three to 10 rotations are
recommended. Nightshades, which can serve as hosts, should also be
"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
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."
"Wireworms are shiny, slender, cylindrical, hard bodied,
yellow-to-brown larvae of the click beetle. Wireworms are a pest of
many different crops. In potatoes they feed on seed pieces soon after
they are planted sometimes causing enough damage to reduce the stand.
Later in the season wireworms feed on the tubers, chewing deep pits
and tunneling into the potato thereby decreasing the quality of the
crop. This injury also favors the development of soil-borne diseases
like Rhizoctonia in the tubers."
"Wireworms are long-lived soil-dwelling insects. Eggs are laid in the
summer, and hatch in the autumn. The larvae (the wireworms) then take
a further four years to complete their development. In the first year
of their life, wireworms are very small (less than 5 mm long) and
cannot do much damage. However, from the second year onwards, they
regularly increase in size and their damage potential rises. In their
final year, they form a pupa in the soil in the autumn and emerge as
an adult beetle the following spring.
Chemical control of wireworms in the potato crop is at best only
partially effective. This means that use of chemicals should be
regarded as a last resort, and that full account needs to be taken of
other cultural methods of control as well.
Avoidance: the best option is not to grow potatoes in a wireworm-infested field.
Cultivation: ploughing and discing will physically damage wireworms
and expose them to predation by birds. On balance, it is probably
better to plough an infested grass field close to planting rather than
in the previous autumn as the decaying turf will act as an alternative
food source for the wireworms. Note that wireworm damage is often
worst in the 2nd and 3rd years out of grass.
Early lifting: wireworm damage tends to steadily increase from about
mid-August onwards, so planting a variety that can be lifted as early
as possible will help reduce the damage risk. There are no potato
varieties with resistance to wireworm attack.
Insecticides: soil insecticides applied at planting (e.g. Mocap,
Nemathroin) only reduce wireworm damage; they never eliminate it
completely, and occasionally don?t work at all. This is why they
should only be used as the last line of defence."
"Q: Is there any way to destroy or overcome the destructive work of
the wireworm, which I find in some spots takes the lion's share of
crops, such as beans, potatoes, onions, etc.?
A: We do not know any easy way with wire worms. Nitrate of soda is
believed to kill or repel them, but you have to be careful with it,
for too much will either over-stimulate or kill the kill; about 200
pounds per acre, well distributed, is the usual prescription for the
good of the plants. Wire worms can probably be killed with carbon
bisulphide, using a tablespoonful poured into holes about a foot deep,
three or four feet apart. The vapor would permeate the soil and kill
all ground insects, but the acre-cost of such treatment must be
measured in its relation to the value of the crop. The most promising
policy with wire worms is rotation of crops, starving them out with a
grain or grass crop and not growing such crops as you mention
continually on the same land."
"The subterranean habits of wireworms make it hard to exterminate them
when they have once begun to attack a crop, and the most hopeful
practice is, by rotation and by proper treatment of the land, to clear
it of the insects before the seed be sown. Passing easily through the
soil on account of their shape, wireworms travel from plant to plant
and thus injure the roots of a large number in a short time. (See
EcoNoMIC ENTOMOLOGY.) Other subterranean creaturessuch as the
leather-jacket grub of crane-flieswhich have no legs, and geophilid
centipedes, which may have over two hundred, are often confounded with
the six-legged wireworms."
Scouting for Wireworm:
"Scouting for wireworms is most easily accomplished with a bait trap.
An illustration of the University of Missouri wireworm trap is shown.
The trap should be in the soil at least 1 week before planting.
Wireworms are more likely to be near the soil surface as the soil
temperature increases in the spring, so scouting close to planting
will increase the likelihood of finding wireworms. The scouting
technique consists of 1/2 cup of a 1:1 corn:wheat seed mixture placed
in a hole and covered with soil. The seed mixture should be soaked in
water 24 hours prior to placement in the hole to facilitate
germination. A black plastic trash bag placed on the soil surface over
the bait will help warm the soil and speed germination of the seed.
The edges of the trash bag should be covered with soil to prevent wind
from blowing it away."
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