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

14 Organic Resource Guide
Cultural Control:
1. A three-year rotation to crops other than Alliums is recommended.
2. For storage, dry bulbs quickly and store in a cool (32 o -36 o F), dry place (See: http://
www.nysipm.cornell.edu/organic_guide/stored_fruit_veg.pdf).
Materials Approved for Organic Production:
At this time, none have shown efficacy.
PINK ROOT (Phoma terrestris)
Pink root affects onions but not other Allium crops. The fungus is a saprophyte and only a
weak pathogen. It usually infects weak or stressed plants or follows injury or infection by
another root pathogen. The hyphae of the fungus invade the roots, and the infected root turns
pink and may shrivel and die. Sometimes infections are limited, and only a few roots show
symptoms. In a severely infected plant, all of the roots are damaged, and leaves will either yellow
and wilt or turn yellow with a red hue. Severe infections can reduce bulb size and yield.
Cultural Control:
1. Crop rotation will not eliminate the fungus from the soil, but repeated onion crops will
lead to more severe pink root.
2. Optimum growing conditions to keep pink root in check include good soil tilth, fertility,
and organic matter; adequate irrigation and management of other pests will also
help.
3. There is some varietal tolerance to pink root. The named sweet onion cultivars
Pegasus, Liberty, and Sweet Melody showed the highest levels of resistance in a trial
conducted in Georgia in 2002. Five numbered varieties showed tolerance higher than
Sweet Melody, so additional tolerant named varieties may be available.
Materials Approved for Organic Production:
At this time, none have shown efficacy.
PENICILLIUM DECAY (Penicillium sp.)
Penicillium decay is a common problem in garlic and onions; however, it rarely causes major
crop loss. Occasionally, the disease reduces plant stand. More often, it is seen on a few bulbs
in storage. In the field, clove decay will lead to wilted, yellowing, or stunted seedlings. Decaying
bulbs in storage may be consumed by white mycelium, which turns blue as spores are
produced (Photo 1.14). Often, the blue mold is only seen on some of the cloves in the garlic
head.
This fungus survives in soil and on all types of plant and animal debris, but these are not major
sources of infection in garlic. Infections primarily come from planting infected cloves and from
plant debris left in the field. Wounding non-infected cloves when cracking diseased heads
apart for planting can lead to infection. In onions, Penicillium invades wounds, diseased tissue,
and freeze-damaged tissue.
Cultural Control:
1. Careful handling of garlic seed pieces is important to reduce wounding at planting
time. Keep the time between cracking heads apart and planting to a minimum.
2. Bulbs should be harvested with care and stored at 32-36 o F.
Materials Approved for Organic Production:
At this time, none have shown efficacy.
DOWNY MILDEW (Pernospora destructor)
Downy mildew occurs only sporadically in the northeast, but disease development can be explosive,
and epidemics can develop rapidly. This disease is caused by a fungus-like water mold
(not a true fungus). The first symptom is pale green to brownish-yellow oval or circular shaped
areas on the leaves, which may appear velvety. Symptoms often appear first on older leaves.