1) SUMMARY
In California, sugarbeet root rot is caused mainly by a few soilborne fungi. These pathogens share very similar life cycles, and their control is based on similar principles. This treatise will consider the two most common pathogens, although the principles are the same for other related pathogens.
Pythium root rot is a serious disease of sugarbeet growing in warm soils. The disease is caused by the fungus Pythium aphanidermatum. The disease occurs in California as well as in other sugarbeet growing areas. Symptoms of the disease include wilting of the foliage and premature death of the lower leaves. Root symptoms are characterized by black necrosis, proceeding inward through the tap root from lateral roots where the infection had begun. A large portion of the surface of the tap root may be discolored. The disease usually progresses downward through the tap root, and secondary organisms may invade the tissue, causing a soft rot. The diseased tissue is usually sharply delimited from the lower, healthy tissue of the tap root.
Phytophthora root rot is caused by the fungus Phytophthera drechsleri. The disease occurs in California when beets are grown during the summer months. The lesions that develop on the surface of the tap root are usually brown. The interior tissue is an amber to reddish brown. Symptoms of the disease usually occur at the tip of the tap root and progress upward toward the crown. A sharp delimitation occurs between the diseased lower tissue and the healthy upper tissue of the tap root.
Both pathogens are zoospore producing oomycetes. Zoospores are motile spores that swim in soil water. Zoospores are attracted to the root surface by exudates produced by the host. If the spore encounters a root surface, it will encyst and infect the tissue. The pathogen grows in the tissues vegetatively and at some point produces more spores. These spores are released in response to water saturation of the surrounding soil, such as would occur during an irrigation event. These secondary spores will infect more tissue and other, nearby roots. The disease progresses with each subsequent irrigation.
Both pathogens require warm soil temperatures for infection. Temperatures above 30º C are conducive for rapid spread of the disease. For this reason, these diseases are rarely seen during the cooler months of the year.
Both pathogens produce resting spores called oospores. These spores are resistant to desiccation and will survive many years in the soil.
Populations of the pathogens are delimited by the soil environment. It is assumed that variability in populations exist both regionally and geographically. This variability has not been studied in relation to sugarbeet root disease, and it is not known how this variabiity affects disease pressure.
Yield loss due to root rot is usually directly related to the amount of disease in a field. Infected beets usually decompose relatively quickly and directly reduce tonnage to be harvested. Loss can also result from more recently infected beets. These have not decomposed and are harvested, but most of the sucrose has been metabolized by the pathogen and secondary organisms. This process results in a very low quality raw product with little if any extractable sugar.
2) CURRENT PEST MANAGEMENT PRACTICES
Cultural Control: Control of Pythium and Phytophthora root rot is difficult at best. Root rot infection occurs by zoospores, and long-term exposure to saturated soil during periods of high temperature must be avoided. Fields should have adequate drainage, and beets should be planted on raised beds. If a grower has a history of root rot, it would be best to grow the beets under sprinkler irrigation rather than irrigating by furrow.
During periods of high soil temperature, furrow irrigation must be practiced
with the utmost precision. Fields must not be allowed to get too
dry. Under this situation, all the feeder roots will die, and when
the field is watered, these roots must first re-grow before the beets are
able to extract water from the soil. This results in a soil profile
that remains saturated for a longer period than necessary. The optimum
irrigation strategy is to water when the soil reaches a critical water
potential, usually between -0.5 and -0.8 bars, then water with as fast
a set as possible. This strategy will keep the beets in good condition
to remove the added water as quickly as possible.
Host Plant Resistance: No qualitative resistance to the rot pathogens
has been found. Continual selection under severe disease pressure
has resulted in the selection of some cultivars better able to withstand
infection pressure than others. These cultivars have been generally
deployed throughout California. Rhizomania resistant cultivars will
have a healthier root system under rhizomania conditions. These cultivars
are generally more tolerant to the rot pathogens than rhizomania susceptible
lines.
Chemical Control: Currently, no pesticides are used for control of root disease caused by these fungi. This is due to lack of fungicides available and to difficulty in reaching the infection sites because of the soil matrix. Pesticides used to control leaf-feeding armyworms and other insect pests may aid in control of root rot by preventing defoliation. This practice will keep the soil cooler and may lower infection pressure. Irrigated sugarbeets that are defoliated transpire less; soils remain wetter longer; and conditions conducive to root rot pathogens are maintained.
Biological Control: Currently, there are no biological options being implemented for rot control.
3) REDUCED-RISK OPTIONS
A new generation of fungicides may become available that could increase the options for rot control. The new class of fungicides called B-methoxyacrylates have been shown to be very effective against the oomycetes as well as other fungal pathogens.
4) CHALLENGES
The cultural controls recommended for root rot control are extremely tricky to practice. Initial infection sites can quickly spread to large areas in a field of disease. This situation can occur with even one error in the irrigation scheduling.
5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM
The new family of fungicides may prove to be very effective in control of rot on sugarbeets. Azoxystrobin (Heritage®) was registered on February 7, 1997, and is a new fungicide for use on golf courses and commercial turf to control Brown Patch, Pythium Blight, and Melting Out (Leaf Spot). It is the first of a new class of pesticide compounds called B-methoxyacrylates which are derived from naturally occurring fungi. They function by inhibiting the electron transport of other potentially competitive fungi. They have low application rates and longer intervals between applications than most alternatives. The broad control spectrum and new mode of action should make it a likely candidate for use in resistance management. It is labeled for use in integrated pest management programs. According to EPA risk assessment, this product has no acute risk levels of concern for birds, mammals, and bees.
6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS
The most likely barrier to implementation of new methods is regulatory.
Registration of new products for sugarbeets has been slow to materialize.
Manufacturers are reluctant to pursue registration of chemicals on minor
crops such as sugarbeets because of the small amount of potential sales.
REFERENCES
Hine, R. B., and Ruppel, E. G. 1969. Relationship of soil temperature and moisture to sugarbeet root rot caused by Pythium aphanidermaturn in Arizona. Plant Dis. Rep. 53:989-991.
Staughellini, M. E., von Bretzel, P., Olsen, M. W., and Kronland, W. C. 1982 Root rot of sugar beet caused by Pythium delieizse. Plant Dis. 66:857-858.
Tomkins, C. M., Richards, B. L., Tucker, C. M., and Gardner, M. W. 1936.
Phytophthora rot of sugar beet. J. Agric. Res. 52: 205-216