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

Cultural Control:
1. Use clean garlic “seed.”
2. Manage irrigation to minimize periods of leaf wetness.
Materials Approved for Organic Production:
None.
PURPLE BLOTCH (Alternaria porri)
Purple blotch is a very common and sometimes destructive disease that affects onions, garlic,
shallots, and leeks. Lesions begin as whitish, sunken areas that elongate and develop purplish
centers (Photo 1.10). Under favorable conditions (i.e., warm with wet leaves), the lesions
become large and oval with concentric rings of dark brown spores. The lesions may merge
and kill entire leaves, which may become covered with brownish spores. Older leaves are more
susceptible than younger leaves. This pathogen may also result in a watery rot at the neck of
onions or garlic and lead to poor storage life.
Onion residue is the source of inoculum in the spring. The fungal mycelia and conidia (spores)
persist as long as onion debris remains in the field or in cull piles. New conidia are produced on
infected tissue in the spring and subsequently wind blown or carried in water to the new crop.
The leaves must be wet for the spores to germinate, but germination is very quick, less than an
hour. Symptoms may appear less than a week after germination, and new spores are produced
quickly.
Cultural Control:
1. Sanitation is very important to limiting spread. Infected crop debris should be
destroyed or buried after harvest. Cull piles should be eliminated.
2. Grow onions in rotation with non-host crops.
Materials Approved for Organic Production:
1. Serenade has been shown to be effective against purple blotch.
2. Copper has shown mixed results in trials, fair at best.
WHITE ROT (Sclerotium cepivorum)
White rot is a very serious problem because it may spread fast, and, once in a field, it can persist
for many years. Luckily, it is a spotty disease that is currently present in only a small number of
fields around the northeast. Those farms can no longer grow Allium crops in infested fields.
White rot is one of the most destructive fungal diseases affecting onion crops, and it is
only damaging to plants in the onion family. It is not the same pathogen as white mold,
which attacks many other crops such as beans, carrots, lettuce, tomato, pepper and others.
Symptoms of white rot on the leaves include premature yellowing and dying of the older
leaves, followed by death of the plant. In garlic, these symptoms are similar to those of some
other diseases and are not particularly diagnostic, but the presence of white, fluffy fungal
growth (mycelia) on the root end of the bulb is the defining characteristic. Eventually, the
fungal growth moves around the bulb and inward between the storage leaves of onion and
cloves of garlic (Photo 1.11). Small, black sclerotia (tiny, hard, black bodies of dormant mycelia)
form in the decaying tissue and throughout the white, fluffy mycelia (Photo 1.12). Secondary
infection by other fungi and bacteria may also occur.
The pathogen is not known to produce spores. This fungus reproduces only by the sclerotia,
and it also spreads by direct contact, i.e., when the mycelium growing on one plant reaches
the roots of the neighbor plant in the row. The sclerotia can lie dormant in the soil for many
years until roots of a host plant grows nearby and the sclerotia are stimulated to germinate (see
below). Transfer of the pathogen can happen on boots and tillage or other equipment. It can
also move with soil during heavy rains. Additionally, animals feeding on diseased bulbs can
defecate viable sclerotia.
12 Organic Resource Guide