Seed treatments, such as Tachigaren, have been shown to suppress early season damping off due to Aphanomyces; however, these must be combined with genetic tolerance to prevent chronic mid-to-late season devastation that can occur in very warm, wet conditions.
Aphanomyces
Description
Aphanomyces or black root rot disease is caused by the fungi aphanomyces cochlioides. Root disease caused by Aphanomyces can occur as two distinct forms in sugar beets.
The early season, or seedling phase, is commonly referred to as black root or acute seedling blight. It does not rot the seed or affect initial stand establishment, but can significantly affect plant stands by inducing damping-off of seedlings several weeks after emergence.
The late season, or root rot phase is characterized by foliage that wilts, turns a dull green color, and eventually becomes yellowed. Losses from the chronic phase are usually less severe than those of the seedling phase.
Symptoms
Acute seedling blight
Aphanomyces can infect sugar beets at any stage of development, but seedlings
are especially vulnerable. Signs of infection usually begin to appear on seedlings
1-3 weeks post-emergence. The hypocotyl develops dark gray, water-soaked lesions.
The lesions quickly spread over the entire hypocotyl, causing it to become
threadlike and turn dark gray brown or black in coloring (image 1). Some diseased
seedlings may fall over and die, while others survive and show some recovery.
Infected seedlings are stunted and exhibit reduced vigor.
Chronic root rot
Infected seedlings that survive, and beets infected later in development,
often exhibit root rot. Abundant lateral roots form, which quickly become
black and shriveled. Occasionally, the fungus will invade the lower portion
of tap root and brown to black lesions appear in that area. This symptom is
designated by the name taproot tip rot. Later, the basal portion of the tap
root may take on a fibrous or tasseled appearance. Aboveground, foliar chlorosis,
wilting and unthrifty top growth can be seen (image 2).
 Image 1 |
 Image 2 |
Development
The causal agent in sugar beets is the water mold fungus, aphanomyces cochlioides. Aphanomyces resting spores, called oospores, can survive for long periods of time in the soil or on infected plant debris. High soil moisture encourages oospore germination. The oospores can either directly infect beets, or produce secondary spores which will then infect new hosts. The secondary spores, called zoospores, are motile and swim towards new hosts. In infected beets, new oospores will form which will lie dormant in soil or plant debri until the next beet crop is planted.
Spread
Under conditions of high soil moisture and free water, oospores germinate
and infect new hosts. The disease intensity is highly dependent on available
soil moisture and soil temperature. Disease intensity increases as soil temperatures
rise from 18 to 32° C Seedlings are seldom infected of the temperature
is below 15° C.
Beet crops planted in cool soil often survive the infection. If the soil cools
down after initial infection, the plants will often recover, but remain stunted.
Under dry soil conditions, older beets can become infected when their tap
root grows into soil zones of higher moisture, causing the tip rot phase.
Control
For control of aphanomyces root rot, early planting into cool soils is recommended. This promotes good emergence and vigorous growth by enabling beets to grown beyond the very susceptible seedling stage before the soil warms up and pathogen activity is enhanced. Minimal irrigation should be applied during emergence. High soil fertility, especially high levels of phosphorus, promote rapid seedling growth and reduce the severity.
Rotation with non-susceptible crops such as maize, soybean, potatoes and small grains helps to reduce the black root severity in subsequent beet crops. Crops such alfalfa, bean, sweetclover and clover increase the disease incidence and should not be planted before a beet crop.
Tachigeren seed treatment is effective in controlling aphanomyces root rot.
