Resource Guide for Organic Insect and Disease ... - Cornell University

Cultural Control:
1. Management begins with using clean seed. Seed produced in arid areas is less likely to
be infested.
2. Hot water treatment is recommended for suspect seed lots (122 o F for 20 minutes).
3. Plow crop debris under immediately after harvest.
4. A two-year crop rotation is recommended to allow crop debris to fully decompose.
Materials Approved for Organic Production:
Copper based materials have been shown to be effective if applied as soon as the first
symptoms are seen.
ASTER YELLOWS
Aster yellows is a disease caused by a phytoplasma, which is a very small bacterium that lacks a
cell wall. Phytoplasmas are obligate parasites (cannot live outside a host plant or insect vector),
and species within this group cause a wide array of diseases, such as coconut lethal yellowing,
peanut witch’s broom and elm yellows. The organism lives in the phloem (cells responsible for
nutrient transport) of plants. The species that causes aster yellows has a wide range of hosts
including lettuce, asters, celery, carrot, and many weeds. Phytoplasmas are transmitted from
plant to plant by insects, mainly leafhoppers.
A characteristic symptom of aster yellows on carrot is yellowing of the veins of young leaves.
The leaves also appear narrower than normal. Yellowing progresses until the entire leaf is chlorotic.
Later, a mass of dwarfed, sickly leaves develops from the crown of the carrot (Photo 9.7).
Diseased plants are more susceptible to bacterial soft rot.
Cultural Control:
Control measures are limited to managing the leafhopper insect vector (see above).
Materials Approved for Organic Production:
There are no materials approved for organic production.
DISEASES CAUSED BY FUNGI AND FUNGUS-LIKE ORGANISMS
ALTERNARIA LEAF BLIGHT (Alternaria dauci)
Carrot seedlings may be infected by Alternaria leaf blight that has overwintered in the soil, on
crop debris, or on seed. The symptoms of seedling infection are similar to damping off. Later in
the season, infections of mature plants may result from wind-blown spores from nearby fields,
crop residue, weeds, or equipment. Moisture on the leaf surface is necessary for spores to germinate,
so the disease is favored by rainy seasons. Foliar lesions first appear on the margins of
the leaflets and begin as greenish brown, water-soaked areas, which later enlarge and turn dark
brown. When much of the leaf is infected, the whole leaf eventually yellows, collapses, and
dies. When lesions form on the petiole, the leaves die quickly. Older leaves are more susceptible
to infection; often the young leaves appear healthy while all of the older leaves are dead
and have collapsed (Photo 9.8).
Cultural Control:
1. Crop rotation is critical if Alternaria leaf blight was a problem the previous year. The
pathogen can survive until diseased tissue decomposes. Burying diseased residue at
the end of the season, and a two to three year rotation is recommended.
2. Growers should use clean seed.
3. Hot water treatment of the seed helps to reduce infection. Soak seed in hot water
(122°F; 50°C) for 20 minutes. Do not soak longer, or seed could be damaged.
4. Use resistant varieties if leaf blight has been a problem in the past.
5. Thin plants to allow good air flow for quicker drying.
6. If there are healthy, young leaves late in the season, even if most of the older leaves
are dead, an application of fertilizer may be enough to encourage carrots to reach
marketable size.
Organic Resource Guide 77