Managing sugarbeet diseases

by Mohamed Khan, Extension Sugarbeet Specialist

Several fungal sugarbeet diseases cause significant production issues in Minnesota. Management of three of the most important diseases will be discussed here: Cercospora leaf spot, Rhizoctonia root rot, and Fusarium yellows.

Managing Cercospora leaf spot

Figure 1. Cercospora leaf spot damage on older leaves of sugarbeet plants as lesions coalesce.

Figure 2. Cercospora beticola kills the mature and productive leaves resulting in re-growth of new leaves, reduced tonnage and significantly lower sugar concentration with higher production costs.

Cercospora leaf spot (CLS) (Figure 1 and 2) is the most destructive foliar disease of sugarbeet in North Dakota and Minnesota. The causal agent of CLS is the fungus Cercospora beticola which does most damage in warm weather (day temperature of 80 to 90° F and night temperature above 60° F) and in the presence of moisture from rain or dew on the leaves. Since the fungus destroys the leaves, it adversely impacts photosynthesis resulting in reduced tonnage and 2-3 percent reduction in sugar concentration.

Growers typically mange CLS by using crop rotation with non-hosts, planting CLS resistant varieties, and timely fungicide applications.

Growers should not allow the disease to become severe before initiating fungicide application since that will be a recipe for disaster. It is recommended that after row closure, fields should be scouted every 5 days so that the first application can be made at first symptoms. Symptoms typically appear first in fields close to waterways, shelterbelts, last year’s sugarbeet fields, and next to corn fields.

The best way to control CLS during the growing season is to apply fungicides in a timely manner. For ground application, apply fungicides in 15 to 20 gallons of water per acre at 75-100 psi pressure; aerial applicators should use 5 gallons of water per acre for best results. Use full rates of fungicides when they are used alone, and a minimum of 0.8 times the labeled rates of each fungicide when using mixtures.

Research shows that application of effective fungicides at first symptoms, and subsequent applications at least at 14 day intervals based on the presence of leaf spots and favorable environmental conditions (Daily Infection Values for two consecutive days of 7 or higher) consistently provided the most effective and economical control. In 2015, when environmental conditions were very favorable for CLS infection and development, research demonstrated that the timely use of different chemistries of fungicides in a rotation program provided effective disease control and did not impact yield nor quality.

Figure 3. Four inner rows of sugarbeet plot treated with fungicides using a mixture of TPTH and Topsin in the first application followed by a triazole and a strobilurin fungicide.

Figure 4. Non-treated check with severe CLS infection and regrowth of new leaves.

Mixtures of effective fungicides should be used especially in the first application to prevent the pathogen from increasing its population. Over the past decade, a mixture of Triphenyltin hydroxide (TPTH) (4L) and Topsin (F) fungicides as the first application followed by other chemistries in a rotation has consistently provided effective CLS control (Figure 3) compared to a non-treated check (Figure 4). Since NDSU’s laboratory data based on samples collected from growers’ fields indicate an increasing number of Cercospora beticola isolates with resistance to triazoles and QoI fungicides, the use of a Topsin + Tin mixture should help to control any overwintering CLS isolates which are fungicide resistant. The triazoles can then be used in rotation with the strobilurins (or QoI), with a fourth application, if necessary in a heavy disease year, in the form of TPTH. Please consult your agriculturists to determine if there are fungicide resistance issues in your township so that a more specific rotation can be developed for your field. In principle, most of the fungicides (TPTH, Topsin, Eminent, Minerva, Minerva Duo, Inspire XT, Proline, Topguard, Headline, Gem, and Priaxor) recommended will provide effective CLS control once they are used in a timely manner and in a rotation (individually or in mixtures) program.

Most fungicides in a liquid form, with the exception of Minerva Duo, can be mixed with glyphosate for control of CLS and weeds. It is recommended to do a compatibility test with each batch of mixtures to ensure that the mixture will not result in nozzle plugging. Research is ongoing to determine the utility of using mixing copper products with other recommended fungicides for controlling CLS and managing fungicide resistance.

Rhizoctonia root rot of sugarbeet

Figure 5.Severe Rhizoctonia root rot of sugarbeet.

The fungus Rhizoctonia solani causes Rhizoctonia damping-off, crown and root rot of sugarbeet in North Dakota and Minnesota. Rhizoctonia is listed as the most important production problem for sugarbeet growers since 2009. Growers are using more Rhizoctonia tolerant varieties coupled with seed treatments to protect from damping-off. The most widely used seed treatment is Kabina which gives about four to five weeks of protection.

It is recommended that growers use a post emergent fungicide to protect plants from crown rot and root rot. Crown rot is initiated when soil with inoculum is thrown into the crown at cultivation, and during heavy rainfall or flooding. Crown rot has not been a major issue with the adoption of Roundup Ready sugarbeet and the concurrent reduction or non-use of cultivation for weed control.

Root rot has been a major issue for growers. Fields with no fungicide seed treatment for control of Rhizoctonia should be treated before the daily average soil temperature at the 4-inch soil depth reaches 65°F and preferably timed just prior to a rainfall (1/4 to 1/2 inch) event. Fields planted with Kabina or Kabina and Rizolex (or Vibrance) treated seeds can be treated with an effective fungicide such as Quadris, Priaxor and Proline when the plants are at the 4 to 6-leaf stage. The fungicide needs to get into the soil and as close as possible to the roots so treatment should be done before the leaves cover too much of the soil. Timing the fungicide application before a half-inch rainfall is ideal to get the fungicide to protect the roots and target the pathogen in the soil. Many growers with early planted sugarbeet have applied a fungicide and have had timely rains to move the products into the soil and surrounding roots which should result in adequate protection. Growers with replanted beets, especially in a field with a history of moderate to severe Rhizoctonia should also make a timely fungicide application to protect against Rhizoctonia root rot.

Fusarium yellows found at research site in Moorhead, MN

Fusarium yellows were first observed in late May and confirmed on June 6 at the NDSU and University of Minnesota research site in Moorhead, MN. The causal agents for Fusarium yellows (and decline) are the pathogenic fungi Fusarium oxysporum and Fusarium secorum. Fusarium yellows may cause significant reduction in plant stand and root yield, and it is recommended that infected roots not be placed in long-term storage.

Figure 6. Fusarium yellows symptoms on 2-6 leaf stage sugarbeet. Note yellowing or necrosis on oldest true leaves first.

Figure 7. Typical Fusarium yellows symptoms on older sugarbeet plants. Note interveinal chlorosis and necrosis starting on oldest leaves followed by similar symptoms on younger inner leaves.

The pathogens may infect seedlings and older plants in fields where soil temperature is at or above 65°F and in the presence of adequate moisture or wet conditions. In seedlings and young plants, oldest leaves become yellow following by wilting and death (Figure 1). Cross sections of infected roots will show darkening of the vascular system. On older plants (4 leaves and older), symptoms include interveinal yellowing and death of older leaves (Figure 2), sometimes distinct necrosis of half the leaf on one side of the midrib which then spreads to the other side of the mid-rib; necrosis of older leaves followed by death of the younger leaves. Under severe disease conditions, infected plants may die with seedling being more vulnerable.

In fields where the disease is not severe, older leaves of infected plants display typical foliar symptoms but the plants survive. Roots of infected plants have roots with no external symptoms, but when these roots are cut in a cross section, there is a distinct darkening and damage of the vascular system. Roots of infected plants do not store well in piles and have high respiration rates which results in low sugar concentration during storage. Manage Fusarium yellows by planting varieties with tolerance to Fusarium yellows and other root diseases common to specific fields. Consult your agriculturists or seed sales representatives for Fusarium tolerant varieties appropriate for your growing area. Fields should be scouted for Fusarium yellows and records should be kept to be used when deciding on varieties the next time that field will be planted to beets again.