Identification and Control of Seedling Diseases, Root Rot, and Rhizomania on Sugarbeet (continued)

PP-1142, BU-7192-S, February 1998

Control Measures for Seedling Diseases and Root Rots
Rhizomania
Imposters: Problems that Resemble Seedling and Root Diseases

Control Measures for Seedling Diseases and Root Rots

Seed treatment fungicides

These products are a convenient, economic, and effective method of reducing seed rot and damping-off. All commercial sugarbeet seed is pretreated with fungicides, and the products used are listed on the seed package. Some fungicides have activity against one pathogen while others have activity against two or more pathogens. Combinations of fungicides are applied to broaden the spectrum of activity and protect against two or more fungal pathogens. The most effective seed treatment fungicides are active against certain fungi (Table 2), so knowledge of which pathogens occur in a field is important to obtain the best stands.

Table 2. Fungicides registered for sugarbeet seed sold in 1998.

	Disease control of specific pathogens*
Chemical	Aphanomyces	Pythium	Rhizoctonia
Captan	No	F-G	P-F
Chloroneb	No	G	G
Hymexazol (Tachigaren)	G-E	E	No
Mefenoxam (Apron XL)	No	E	No
Metalaxyl (Apron)	No	E	No
Oxadixyl (Anchor)	No	E	No
Pentachloronitrobenzene (PCNB)	No	P	G
Thiram	No	G	F

* P = Poor; F = Fair; G = Good; E = Excellent; No = No control.

Pythium species occur in all fields and are active at the same temperatures favorable for beet seed germination. Thus, fungicides with specific activity against Pythium are particularly important in protecting seed. Excellent control of Pythium is achieved with several fungicides, but the most widely used product is Apron (= metalaxyl). If seed has not been treated with a fungicide, Apron Dry Seed Protectant (12.5%) can be applied in the drill box, but thorough mixing of fungicide and seed is essential for good control.

Aphanomyces damping-off is reduced by seed treatment with Tachigaren (hymexazol). It is registered for application to pelleted seed at rates of 45 to 90 grams per unit of 100,000 seed (approximately 2.2 pounds). Only the 45 and 75 g rates are available for the 1998 season. Tachi-garen also is highly effective against Pythium at lower rates than those recommended for control of Aphanomyces. For low to moderate levels of Aphanomyces, a label rate of 45 grams per unit is recommended. The 75 g rate is intended for fields with moderate to severe disease. The 90 g rate, if available, should be reserved for use only in the most severely infested fields. Tachigaren loses effectiveness as it decomposes and provides protection to seedlings for about three to four weeks.

Seed treatment fungicides provide limited protection of seedling stands because they decompose from one to four weeks after planting, depending on the fungicide. The greater the plant population that emerges, the greater the number of seedlings that survive. Overseeding may not compensate for stand losses and risks buildup of pathogen populations. Pythium usually is effectively controlled by seed treatment because the fungus is active early in the season. Aphanomyces cochlioides and Rhizoctonia solani, however, can cause problems throughout the season. In fields with a history of disease pressure, particularly with Aphanomyces, use of an appropriate fungicide on varieties with partial resistance is recommended.

Control tips: Aphanomyces

• Select varieties with partial resistance
• Plant Tachigaren-pelleted seed
• Plant early
• Cultivate and keep soil dry
• Enhance field drainage
• Increase length of rotation
• Control weeds
• Avoid spread of contaminated soil

Soil treatment fungicides

Various formulations (granule, emulsifiable concentrate, wettable powder) of Ridomil (metalaxyl) are available to supplement Apron (metalaxyl) seed treatment for control of Pythium species. Yield increases are documented in producers' fields, but benefits vary among fields and seasons. These variable results are attributed to different populations of Pythium in fields and because cool, wet soil conditions (favorable for Pythium activity) do not occur each spring. Ridomil is beneficial when sugarbeet seeds are planted into cold, wet fields with a history of Pythium disease problems. Check the label for planting restrictions within 12 months of application.

Control tips: Pythium

• Select seed treated with an appropriate fungicide
• Plant early
• Avoid deep planting in wet soil
• Cultivate to keep soil dry

Planting date

Pythium aphanidermatum, Aphanomyces cochlioides, and Rhizoctonia solani do not grow well at low temperatures, so infection of sugarbeet seedlings by these pathogens can be avoided by planting early, into cool soils. Early planting fosters good emergence and vigorous growth. This enables plants to advance beyond an extremely susceptible stage before soils warm up and pathogen activity increases. Infection by P. ultimum cannot be avoided by altering planting date since the fungus is active at the same temperatures favorable for germination of sugarbeet seed. Seed treatment with fungicides with specific activity against Pythium species is recommended.

Planting depth and soil moisture

Pythium species and A. cochlioides require wet soil to allow swimming zoospores of these fungi to move through soil and infect sugarbeet roots. Shallow planting at a �-inch depth encourages maximum emergence and reduces disease in wet, early-seeded fields. Cultivation helps dry out soil and reduces early season losses from Aphanomyces seedling diseases. R. solani has a lower requirement for soil moisture and is less affected by planting depth than Pythium species and A. cochlioides.

Rotation

Crop sequence affects seedling diseases and root rots. The relationship of crop sequence and disease severity, however, is not the same for all sugarbeet pathogens.

Pythium species attack roots of all agricultural crops and weed species and survive in soil for years. Germination of resting spores of Pythium species is stimulated by seed and root exudates, particularly when soils are wet. The fungus then infects young succulent roots, produces resting spores, and increases its population. Populations of Pythium species are as closely tied to environmental factors in soil as they are to previous crops. Duration of rotation has no effect on Pythium species because of their wide host range.

Crops rotated with sugarbeet are not known to be hosts of Aphanomyces. Some crops, however, are reported to decrease Aphanomyces damping-off and root rot if grown before sugarbeet. Unfortunately, evidence for the effect of previous crops on Aphanomyces diseases is conflicting. Researchers in Michigan reduced damping-off by soil-incorporation of corn residue. Severe damping-off caused by A. cochlioides, however, has been observed in sugarbeet fields in southern Minnesota where the previous crop was corn. Recent data suggest that chisel plowing a green oat crop into the soil in late fall may reduce Aphanomyces diseases of sugarbeet the following season, but results are best when disease pressure is low to moderate.

Increasing the number of years between growing a sugarbeet crop has a limited effect on reducing populations of A. cochlioides because the fungus produces thick-walled oospores that survive in soil for years. Effectiveness of rotation depends on the initial population of the fungus. Long rotations in fields with low populations of A. cochlioides slow down buildup of inoculum. On the other hand, severe Aphanomyces root rot has been observed 20 years after severely infested fields were rotated out of sugarbeet.

Rhizoctonia solani infects many species of crops (Table 1) and weeds and also colonizes organic matter in soil. When a susceptible crop is grown during the rotation, particularly the season before planting beets, benefits of disease control by crop rotation can be lost. This is because some crops (such as soybean and edible bean crops) are susceptible to R. solani AG-1, AG-2-2 and AG-4, which also cause damping-off and root rot on sugarbeet. Cereals are less likely than broadleaf crops to sustain inoculum of R. solani. A three year rotation (two years of nonhost crops) is the minimum duration recommended to allow the fungus population to decrease.

Control tips: Rhizoctonia

• Select varieties with partial resistance
• Plant seed treated with an appropriate fungicide
• Plant early
• Cultivate and keep soil dry
• Avoid "hilling" soil on beet crowns
• Increase length of rotation (minimum of three years)
• Rotate with nonhost crops
• Control weeds
• Avoid spread of contaminated soil

Tillage

Cultivation to encourage soil drying or deep tillage to promote better water penetration reduces seedling diseases caused by Pythium species, A. cochlioides, and R. solani. Other practices that divert excess moisture, such as drainage ditches or tiling, also reduce seedling diseases and root rots. Reduction of soil moisture is beneficial in controlling R. solani, but this fungus also infects roots when soil is somewhat dry.

No-till of small grain crops the season before a sugarbeet crop is planted produces environmental conditions favorable for seedling diseases and root rot. The thick layer of straw retains soil moisture and stabilizes soil temperature - environmental conditions optimal for repeated infections of sugarbeet roots by pathogens.

Rhizoctonia solani AG-2-2 usually infects adult sugarbeet roots through petioles and crowns. High speed cultivation deposits "hills" or excess soil around beet crowns (Figure 16). If R. solani is present in soil hilled on the beet crown, the fungus can readily infect. The sugarbeet canopy also provides a moist and warm microclimate favorable for infection by R. solani. Cultivation at moderate speeds results in less soil deposition in the beet crown and consequently, less Rhizoctonia root and crown rot.

Weed control

Some common weeds, including pigweed, kochia, and lamb's-quarters, are infected by Aphanomyces cochlioides and Rhizoctonia solani. Control of these weeds during the sugarbeet season as well as during production of other crops discourages buildup and maintenance of these pathogens.

Sanitation

To reduce movement and spread of A. cochlioides and R. solani, equipment used in an infested field should be thoroughly washed in soapy water and cleaned by a high pressure sprayer before it is moved to a healthy field. Used machinery from other beet-growing areas should be washed before introduction into local sugarbeet fields. Also, soil should be removed from boots, tools, vehicles, and all materials contaminated by Aphanomyces-infested soil. Tare soil should be deposited in areas or fields not planted to sugarbeet.

Plant resistance

Commercial varieties with partial resistance to Aphanomyces root rot or Rhizoctonia root and crown rot are available as "specialty varieties." There is no single variety, however, with resistance to both diseases. Nor are there any varieties immune to infection by A. cochlioides or R. solani. When the pathogen is present and weather conditions are favorable for disease, varieties with partial resistance outyield other varieties. Under disease-free conditions, varieties with partial resistance to root rot yield comparably to other varieties.

Root rot fungicides

There are no fungicides currently registered in the United States to control Rhizoctonia root and crown rot, although some experimental fungicides show promise. There are no fungicides registered to control Aphanomyces root rot of older plants, nor are there any experimental fungicides on the immediate horizon.

Rhizomania

Rhizomania occurs in young and adult plants. Expression of symptoms on foliage and roots varies. Some infected plants appear healthy while others have mild to severe symptoms. Early infections can cause severe stunting and yield loss, while late infections may go undetected and cause little or no yield loss. Rhizomania is not related to Rhizoctonia root rot of sugarbeet.

Aboveground symptoms appear as patches of plants with poor growth and light green or yellow-green foliage, similar to nitrogen deficiency. Leaves are narrow, with long and erect petioles (Figure 19). Foliage may become flaccid and wilt without discoloration. Affected plants often occur in lens-shaped patches (Figure 20), and sometimes an entire field shows symptoms. Since rhizomania-infected plants are stunted, weeds tend to be common in the affected portion of the field.

Figure 19. Yellow leaves typical of rhizomania. Narrow, upright leaves with long petioles are characteristic of the disease but may not occur on rhizomania-infected plants. (112KB color image)

Figure 20. Field symptoms of rhizomania. Note yellow lens-shaped areas in front of trees and yellow plants in foreground. (103KB color image)

Below ground symptoms of this disease explain the name rhizomania, also known as "crazy root" or "root madness." Symptoms include stunted taproots with masses of hairy, secondary roots along the sides and tip of the root, giving it a "bearded" or "whiskered" appearance (Figure 21). Roots often appear constricted a few inches below the soil surface and have a "wineglass" shape. A pale yellow to dark brown discoloration of the vascular bundles occurs near the tip of the taproot (Figure 22).

Figure 21. Proliferation of lateral roots results in a "bearded" appearance of the taproot. (87KB color image)

Figure 22. Root constriction results in a wineglass shape. Note internal vascular discoloration near the root tip. (73KB color image)

Fields severely damaged by rhizomania have greatly reduced tonnage, a low percentage of sucrose, and are low in nitrate-nitrogen. The unusual combination of low sucrose and low nitrate is characteristic of fields with rhizomania.

Rhizomania is difficult to diagnose based on symptoms. Suspect plants should be confirmed by laboratory analysis. Positive identification can be done only by sophisticated laboratory tests using serological techniques. To collect samples, dig up plants suspected of having rhizomania and take care to preserve all secondary roots. Then, ship the plants overnight to a laboratory that specializes in rhizomania identification. Sugarbeet agriculturists can assist in collecting samples and sending them to an appropriate laboratory.

Control tips: Rhizomania

• Select resistant varieties (available by 1999)
• Plant early
• Cultivate and keep soil dry
• Enhance field drainage
• Avoid short rotations
• Avoid spread of contaminated soil
• Clean equipment, tools, and boots of contaminated soil before entering rhizomania-free fields
• Plant cover crops to prevent erosion

Biology of Rhizomania

Rhizomania is caused by the beet necrotic yellow vein virus (BNYVV), which is spread by the soil fungus vector Polymyxa betae. The fungus survives in soil for 15-20 years as thick-walled resting spores called cystosori. The BNYVV survives in the cystosori. Cystosori are stimulated to germinate when in proximity to sugarbeet roots and when soil conditions are warm and wet. If the germinating cystosori contain BNYVV, the swimming zoospores that are released also contain the virus. BNYVV is introduced into sugarbeet when zoospores infect root hairs. The fungus then invades the lateral roots, which are killed by BNYVV, and more roots form. If wet weather persists, zoospores are liberated from infected roots and additional infection cycles occur. A mass of hairy roots forms following repeated infections, with much of the mass composed of dead roots (Figure 21).

Several conditions must occur simultaneously for rhizomania to develop: P. betae is present in the field; BNYVV is associated with P. betae; soil temperatures are above 59 F (most infections occur at 77 F); and the soil is wet (a condition necessary for zoospore production). Rhizomania tends to occur most frequently in lower portions of fields, areas with poor drainage, in compacted soils, and along hillsides where water seeps to the soil surface.

Control measures for rhizomania

There are no seed treatments that control this disease. Every effort should be made to avoid introduction of rhizomania-contaminated soil into geographic areas where it has not been reported. Even small amounts of contaminated soil (such as one teaspoon) can potentially result in a significant rhizomania problem after growing sugarbeet crops for a few seasons. Equipment from infested areas should not be moved to noninfested areas. If equipment is moved from a rhizomania-infested area, it should be power-washed with soapy water and steam-cleaned. All tools and vehicles employed in infested areas should be thoroughly cleaned before entering rhizomania-free areas. Also, boots worn in infested areas should be thoroughly cleaned or worn only in infested fields.

In geographic areas where rhizomania already exists, all tools and equipment should be cleaned when moved from an infested to a noninfested field. Infested fields should be harvested last. Prevent wind and water soil erosion by planting cover crops. Cultural practices, such as early planting to avoid early infection, tiling to improve drainage, and deep tillage to improve water penetration, also help avoid serious losses from rhizomania. Tare soil from infested fields should not be returned to noninfested fields. Extending crop rotations is of little benefit, but avoidance of short rotations prevents buildup of inoculum to damaging thresholds.

Soil fumigation has been used in California to reduce rhizomania infections and to delay the onset of infection. At present it is unclear if fumigation is economic in nonirrigated sugarbeets. Research on soil fumigation of nonirrigated sugarbeet land is under way to determine if treatment of severely infested fields or portions of fields is practical.

Varieties with resistance to rhizomania that are adapted for sugarbeet production in Minnesota and North Dakota should be available by 1999. Until resistant varieties are available, producers should not plant a sugarbeet crop in infested fields.

Imposters: Problems that Resemble Seedling and Root Diseases

Diagnosis of seedling diseases and root rot can be confusing because several other disorders produce symptoms that resemble these diseases. The disorders include a wide variety of problems including wind injury, heat, excess soil moisture, frost, insects, insecticides, soil fertility problems, and herbicide drift and carryover. When evaluating the possible cause of a problem, consider the past and present history of climatic, cropping, and field conditions. Some examples of impostors of seedling and root diseases are illustrated in Figures 23-32.

Root maggot

Root maggot damage on sugarbeet occurs as larvae feed. Larvae scrape the root surface with their mouth hooks and cause irregular scars, which later darken when sap exudes from the damaged root. Larvae feeding on small tap roots can sever the root (Figure 23, top), causing a sudden and permanent wilting of foliage sometimes confused with root rot. On larger roots, larvae produce irregular scars (Figure 23, bottom). Presence of the root maggot is confirmed by carefully removing and examining soil around the root for small white larvae.

Figure 23. Scars caused by feeding of the sugarbeet root maggot on young root (top) and older root (bottom). (137KB color image)

Root aphid

The sugarbeet root aphid sucks sap from roots and reduces the quality and size of beet roots. Heavy infestations result in wilting and death of sugarbeet plants (Figure 24). Affected roots are characterized by a white waxy material secreted by aphids on the small lateral roots and lateral grooves of the taproot (Figure 25). This material remains even after aphids have left the root.

Figure 24. Wilting and death of foliage from sugarbeet root aphids sucking sap from roots. (137KB color image)

Figure 25. A white waxy material deposited by aphids is present on roots, even after aphids have left. (97KB color image)

Insecticides

Modified in-furrow application of Counter (terbufos) into light-textured soil can result in brown constriction of seedling roots at the point of seed attachment (Figure 26). The root system has a corkscrew or coiled appearance, tips of cotyledons turn brown or black, and severely affected plants die. Careful examination of soil around affected young seedlings will reveal granules of insecticide close to the seed. Severely affected seedlings may die before emergence.

Figure 26. Counter injury on sugarbeet seedlings starts as a darkened band near the point of seed attachment. (53KB color image)

Modified in-furrow applications of Lorsban (chlorpyrifos) can result in wilting. Affected roots are constricted about 1 inch below the soil surface with no rot extending above or below the weakened area (Figure 27). These weakened plants are prone to wind damage.

Figure 27. Lorsban injury weakens roots and plants are prone to wind injury. (46KB color image)

Herbicides

A herbicide applied at the recommended rate in a previous season may not completely decompose because of weather conditions and soil type. Sugarbeet seedlings are sensitive to residues of certain herbicides in soil. For instance, residues of dinitroaniline herbicides such as Treflan (trifluralin), result in stunted sugarbeet seedlings. Roots turn brown and die starting at the point where the root joins the hypocotyl, about 1 to 1� inches below the soil surface (Figure 28). Knowledge of herbicides applied at least the previous two seasons, soil type, and soil moisture conditions can help identify herbicide carry-over problems. Herbicide carry-over occasionally persists six to seven years after application.

Figure 28. Treflan carry-over injury on sugarbeet. Root is brown and shriveled at the same depth on affected seedlings. (54KB color image)

Drift from certain herbicides not only affects sugarbeet foliage but can produce injury that resembles root rot. For example, when sugarbeet plants are exposed to imidazolinone or sulfonylurea herbicides, foliage turns bright yellow and, if severely affected, roots become brown and constricted. Sugarbeet plants shown in Figure 29 were exposed to Harmony, a sulfonylurea herbicide. Root rot is distinguished from Harmony damage by symptoms on foliage – Harmony damage is characterized by a bright yellow color on young leaves whereas early symptoms of root rot include mild yellowing of lower leaves and wilting.

Figure 29. Harmony injury on sugarbeet is characterized by bright yellow foliage and by constriction and browning of roots. Severely damaged plants die. (52KB color image)

Wind

Sugarbeet seedlings are particularly vulnerable to wind injury. Damage is associated with the back and forth oscillation of plants in wind and exposure to blowing soil, which acts as an abrasive and shears off young plants. On very young seedlings it may be impossible to determine if stands were reduced by wind or disease. If the weather has been hot and dry and accompanied by strong winds, however, wind damage is probable. If weather has been wet, seedling disease is more likely.

On older roots, wind damage produces a sudden wilting and death of foliage similar to aboveground symptoms of root rot. The cause of plant death is revealed by careful examination of the root. Wind damage causes severe root constriction about an inch below the soil surface, but the root appears normal and healthy above and below this point (Figure 30). If the root has a black rot on the petioles or crown, it likely was killed by Rhizoctonia. If the root is thin and threadlike, it likely was killed by Aphanomyces. Wind injury may be confused with root damage caused by the insecticides Lorsban and Counter.

Figure 30. Sugarbeet plant twisted in half by strong winds. Note the normal color on root, above and below the break. (104KB color image)

Water

When sugarbeet fields are flooded or heavily saturated for several days, oxygen movement to the root ceases and plants die. Sudden permanent wilting of foliage can be confused with Aphanomyces or Rhizoctonia root rot. Roots affected by excess moisture are characterized by a slimy rot and disintegration of the root by nonpathogenic microorganisms (Figure 31).

Figure 31. Slimy decomposition and rot of roots resulting from prolonged soil saturation. (58KB color image)

During drought conditions, foliage cannot extract enough moisture from soil to meet transpiration needs of the plant. Initially, this condition results in temporary wilting, but if prolonged, older leaves die prematurely (Figure 32). Foliage of plants already affected by root rot or rhizomania wilt more readily than healthy plants. Drought conditions, however, are not favorable for infection by soilborne fungal pathogens.

Figure 32. Plants wilt and older leaves die prematurely during prolonged drought. (120KB color image)

For additional information on diseases and other problems on sugarbeet, see: Compendium of Beet Diseases and Insects, 1986, by E.D. Whitney and J.E. Duffus, eds., (available from The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121, $39.00 including shipping; outside of U.S.A., $44.00 surface mail, $48.00 air mail) and Herbicide Mode of Action and Sugarbeet Injury Symptoms – A-1085, 1994, by A.G. Dexter et al. (available from North Dakota State University Extension Service, Fargo, ND 58105, $1.50 including shipping).

Photo credits:

Figures 1-3, 5-12, 15, 17, 18, 21, 22, 26, 28 and 29: photos by C.E. Windels, Northwest Experiment Station, University of Minnesota, Crookston, MN

Figure 4: data from P. Payne and M. Asher. 1989. British Sugar Beet Review 57:44-47.

Figures 13, 30 and 31: photos by C.M. Rush, Texas Agricultural Experiment Station, Bushland, TX.

Figures 14 and 16: E.G. Ruppel, USDA-ARS, Crops Research Lab, Fort Collins, CO.

Figure 19: H.A. Lamey, Department of Plant Pathology, North Dakota State University, Fargo, ND.

Figures 20 and 32: A.W. Cattanach, Department of Soil Science, North Dakota State University, Fargo, ND.

Figure23: R. Dregseth, Department of Entomology, North Dakota State University, Fargo, ND.

Figures 24 and 25: Chris Campbell, formerly of the Department of Entomology, University of Minnesota, St. Paul, MN.

Figure 27: R.A. Kuznia, formerly of the Northwest Experiment Station, University of Minnesota, Crookston, MN

Use of tradenames or mention of commercial products or manufacturers is for information purposes only and does not imply endorsement by North Dakota State University.

Additional copies of this item can be ordered from the NDSU Extension Service, PO Box 5655, Fargo, ND 58105 and the University of Minnesota Extension Service Distribution Center, 20 Coffey Hall, 1420 Eckles Avenue, St. Paul, MN 55108-6069, email: order@dc.extension.umn.edu or credit card orders at 800-876-8636 or 612-624-4900 (local calls).

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PP-1142, BU-7192-S, February 1998