|
Click on images |
 Figure 1. Pod Lesions. |
 Figure 2. Closeup of external pod lesion showing sporulation. |
 Figure 3. Internal pod decay and seed infection (insect). |
Authors
B.A. Edmunds, Graduate Student, Plant PathologyG.J. Holmes, Extension Plant Pathology
Introduction
Anthracnose on common bean occurs throughout the world and is especially devastating in temperate and subtropical areas. Yield losses can reach 100% if the disease is not managed.
Symptoms/Signs
Anthracnose affects all above ground parts of the plant including leaves, petioles and pods. Seedlings can also be affected. Lesions on the leaves and petioles can occur on both sides of the leaf surface and may appear to be elongated and/or form along the leaf veins. The lesions start as reddish-purple specks that progress into brown-black lesions. Symptoms on pods are reddish rust colored lesions or specks that develop into sunken tan colored lesions surrounded by black rings. Lesions on pods range from 1-10 mm in diameter. In humid weather masses of tan or pink colored spores may form in the center of the lesions.
Causal Organism
Anthracnose on beans is caused by the fungus Colletotrichum lindemuthianum.
Disease Cycle
C. lindemuthianum overwinters in crop debris and conidia are spread through seed, wind and rain. C. lindemuthianum is also seedborne. The temperature range for infection is 55-80°F and high relative humidity (>90%) is also required.
Diagnostic Methods
In the field: Look for characteristic lesions as described in the symptoms section. In moist weather, a pink mound of spores may be present in the center of the lesion and can be seen using a hand lens.
In the lab: Look for lesions as described in the symptoms section. Place sample in moist chamber to induce sporulation within the lesions. Wet-mount spores onto a microscope slide. C. lindemuthianum conidia are unicellular (no cross walls), clear to light tan, and cylindrical with rounded ends or with a narrow base). Size range for conidia is 2.5-5.5 X 9.5-22 µm.
Disease Management
- Resistant cultivars of common bean are available. Check with seed companies for the most current varieties. The NCSU Horticulture Department maintains a list of bean cultivars and their disease resistance (including anthracnose resistance): Cultivar names starting with A-L http://cuke.hort.ncsu.edu/cucurbit/wehner/vegcult/beangrnal.html Cultivar names starting with M-Z: http://cuke.hort.ncsu.edu/cucurbit/wehner/vegcult/beangrnmz.html.
- Use clean seed; anthracnose can be seed-borne.
- If anthracnose is already established in the field, a 2-3 year rotation away from beans and other legumes is recommended. Corn and solanaceous crops (tomatoes, potatoes, eggplant, etc) are possible non-host rotation crops. Bury infected debris to avoid spreading the disease in the following season. Mulch to prevent soil from splashing onto plants during watering.
- Effective foliar fungicides are available and, if used, should be applied during flower set, late flowering, and at pod fill to control symptoms to reduce direct yield losses. Consult the North Carolina Agriculture Chemicals Manual: http://ipm.ncsu.edu/agchem/agchem.html (Refer to the section on vegetable diseases and table titled 'Relative effectiveness of various chemicals for foliar disease control in beans.')
Other Resources
Schwartz, H.F., Steadman, J.R., Hall, R., and R.L. Forster. 2005. Compendium of bean diseases. APS Press, St. Paul, MN.** August 2007