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

Materials Approved for Organic Production:
Copper fungicides can be effective in limiting secondary spread. See the copper fact sheet
(page 123) for a discussion of efficacy and minimizing accumulation in the soil.
DISEASES CAUSED BY FUNGI AND FUNGUS-LIKE ORGANISMS
ROOT ROTS
There are a few soil borne pathogens that cause root rots and have very similar aboveground
symptoms because they result from lack of root function. Leaves yellow, and the plants are
stunted and slowly die. These species may attack young peas or beans and cause damping-off,
seed decay, and wilts.
FUSARIUM WILTS (Fusarium oxysporum f. sp. pisi on peas, and F. oxysporum f. sp.
solani on beans)
Fusarium is a root-rotting fungus that survives in the soil as very resistant spores that can
persist without a host for more than 10 years. Both peas and beans are susceptible. The
fungus penetrates the roots and invades the vascular tissue of the plant. Early symptoms are
downward turning leaves, followed by yellowing of the plant from the base to the tip. The root
system superficially looks normal, but slicing the roots or base of the stem longitudinally will
reveal a reddish brown discoloration of the vascular tissue. Warm, dry conditions cause the
disease to progress, rapidly killing the plants. Planting peas in the earliest workable fields is
recommended, so the crop develops during the cool part of the season. Optimum growth of
the Fusarium fungus occurs when the soil warms to 68 o F – 72 o F.
PYTHIUM DISEASES (Pythium spp.)
There are several species of Pythium that cause pre- and post-emergent diseases that effect the
seeds (seed rots), seedlings (damping off), and roots (root rots) of peas and beans. Once the
plant emerges and develops mature cells with thicker cell walls, Pythium is less likely to be a
problem. It may still attack young cells at the root tips and lead to “root pruning” and stunted,
chlorotic plants that yellow from the bottom up and may eventually die. At advanced stages,
determining if the plant has died from a wilt or a root rot is difficult. Plants dying from root rot
tend to be easily pulled from the ground. Pythium diseases of seeds and seedlings result in
either gaps in plantings where seeds have simply rotted away or stunted seedlings that died
shortly after emerging.
Pythium spp. are common soil inhabitants with a wide host range, which makes crop rotation
less effective. The pathogen can survive for many years with no host at all or in root debris.
Pythium is a fungus-like water mold (not a true fungus), and high soil moisture is necessary for
it to become a problem for peas or beans.
RHIZOCTONIA ROOT ROT (Rhizoctonia solani)
Rhizoctonia root rot is often lumped together with problems caused by Fusarium and Pythium
because telling them apart in the field is difficult. Seeds, seedlings, and older plants may be
affected, although the pathogen is less likely to attack mature plants. The most distinguishing
symptoms are elongate, sunken, reddish lesions on the hypocotyl, the part of the stem
between the root and seed leaves. The hypocotyl may be girdled. Damping-off of seedlings
and stunting of older plants may occur.
Rhizoctonia overwinters as sclerotia or as mycelium in plant residues. It may be carried on seed
but is more commonly maintained and spread with infected soil. Rhizoctonia produces no
asexual spores and, in fact, rarely produces any kind of spore. Living on decomposing organic
matter, it can remain in the soil for long periods as sclerotia or as mycelium on a wide range of
hosts.
46 Organic Resource Guide