Comparison of Cercospora and Bacterial Leaf Spots on Sugarbeet
PP-1244, April 2003
Carol E. Windels, Plant Pathologist,
University of Minnesota, Northwest Research and Outreach Center
Carl A. Bradley, Extension Plant Pathologist, North Dakota
State University
Mohamed F. R. Khan, Extension Sugarbeet Specialist, North Dakota
State University/University of Minnesota
Click here for an Adobe Acrobat PDF file suitable for printing. (124KB)
Cercospora Leaf Spot (Cercospora beticola)
Economic damage: Commonly occurs, can result in considerable loss in yield and quality and reduces storability of sugarbeet roots in piles.
Cercospora
leaf spots are circular, about 1/8 to 3/16 inch in diameter, with light to dark
tan centers and dark-brown to reddish-purple borders. Elliptical lesions may
occur on leaf blades, veins, and petioles. (Click here
to link to a 27KB color photo of cercospora leaf spots.)
Stromata
(black dots about the size of pepper grains) form during humid weather in leaf
spots on sugarbeet debris or newly infected leaves; they are easily seen with
a hand lens. (Click here to link to a 38KB color photo
of stromata.)
When
weather is warm and wet, stromata produce silver or steel-gray spores which
gives the leaf spot a fuzzy appearance. (Click here
to link to a 34KB color photo of stromata spores.)
Leaf
spots coalesce and kill large areas of leaf tissue. Severely diseased leaves
wither and die, but remain attached to the crown. (Click
here to link to a 39KB color photo of cercospora leaf spot on sugarbeet leaves.)
Sources and spread of inoculum
- Old, partially buried sugarbeet debris from previous crop
- Infected sugarbeet plants during field season
- Common weed hosts such as lambsquarters, pigweed, mallow, bindweed, crops related to sugarbeet (table beet, Swiss chard, spinach) and most wild Beta species
- Spreads via wind, water, and insects
Conditions for disease
- Favored by warm, humid, rainy weather
- Cercospora spores form most readily at 68-79°F at relative humidities of 90-100% (spores do not form at temperatures less than 50°F).
- Spores germinate and infect leaves through stomata (natural openings) at daytime temperatures of 77-95°F, night temperatures above 60°F, and high relative humidities (90-95%) or free moisture. Infection is reduced or inhibited at temperatures less than 59°F or when leaves are wet for less than 11 hours.
- Leaf spots develop from 5 to 21 days after infection, depending on amount of inoculum, temperature, and duration of wet period. Leaf spots typically occur first on lower, older leaves and progress to younger leaves.
Disease management
- Avoid planting within 100 yards of an infected sugarbeet field from the previous year.
- Bury sugarbeet leaf debris by fall tillage.
- Plant to nonhost crops for at least two successive seasons
- Plant tolerant sugarbeet varieties.
- Apply fungicides judiciously (monitor for disease and conditions favorable for disease).
- Alternate different classes of fungicides to avoid development of resistant strains of Cercospora (see current Sugarbeet Production Guide, Cooperative Ext. Serv., North Dakota State Univ., and Univ. Minnesota).
Bacterial Leaf Spot (Pseudomonas syringae pv. aptata)
Economic damage: Commonly occurs but usually not of economic importance; some rhizomania-resistant varieties have shown increased susceptibility to bacterial leaf spot.
Bacterial
leaf spot produces irregular-shaped to circular spots measuring 3/16 to 1/4
inch in diameter. Note tan to dark brown centers with very dark to nearly black
borders. (Click here to link to a 32KB color photo of
bacterial leaf spots.)
No
stromata (black dots) form in bacterial leaf spots; soil particles can lodge
onto leaf spots, so brush lesions gently to remove loose debris. (Click
here to link to a 29KB color photo of a bacterial leaf spot closeup.)
Bacteria
also enter at the leaf margins (initially may appear water-soaked) and leaf
edges turn yellow and then necrotic. (Click here to
link to a 24KB color photo of bacterial leaf spot on the leaf margins.)
Bacterial
leaf spots coalesce between leaf veins; this tissue tears easily and is ragged
in appearance. (Click here to link to a 30KB color photo
of bacterial leaf spot on sugarbeet leaves.)
Sources and spread of inoculum
- Old, partially buried sugarbeet debris from previous crop
- Newly infected sugarbeet plants
- Other hosts include bean, eggplant, lettuce, and pepper
- Infected sugarbeet seed (results in seedling damage)
- Bacterial inoculum spread by splashing rainfall, mechanical and insect injuries
Conditions for disease
- During cool rainy weather, leaf spots usually develop in young plants and before canopy closure; disease may occur throughout the season.
- Infections occur above 36°F and below 95°F; optimal temperatures are between 77-86°F.
- Bacteria enter stomata (natural openings on leaves) or through wounds and injuries caused by hail or wind damage, insects, farming practices, etc.
- Bacteria also enter margins of leaves through hydathodes (natural openings).
- Bacterial leaf spot may be intermixed with Cercospora leaf spot -- on the same leaf or even in the same leaf spot.
Disease management
- No effective field controls have been developed.
Photo credits
Figures provided by H.L. Bissonnette, W.M. Bugbee, M.F.R. Khan, and C.E. Windels.
Selected references
Franc, G.D., R.M. Harveson, E.D. Kerr, and B.J. Jacobsen. 2001. Disease management. Pages 131-160 in: Sugarbeet Production Guide. R.G. Wilson, S.A. Smith, and S.D. Miller (eds.) Univ. Nebraska Coop. Ext. EC01-156.
Whitney, E.D. and J.E. Duffus (Eds.). 1986. Compendium of Beet Diseases and Insects. APS Press, The American Phytopathological Society, St. Paul.
For more information on this and other topics, see: www.ag.ndsu.nodak.edu
