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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Young bean plants killed by Rhizoctonia.

Dry Beans

Seedling Diseases

Pathogens:
Pythium spp., Rhizoctonia solani, and Thielaviopsis basicola

(Reviewed 8/07, updated 8/07)

In this Guideline:

SYMPTOMS

Pythium spp. usually cause preemergence rot and in some cases damping-off of young seedlings. Symptoms include water-soaked lesions with eventual collapse of the hypocotyl at or below ground. Occasionally, older plants are infected and develop water-soaked lesions that extend some distance up the stem, causing a linear band of dead cortical tissue.

Rhizoctonia solani causes postemergence damping-off of the seedlings that is characterized by sharp-edged oval to elliptical reddish brown lesions on the hypocotyl. Heavy infection may girdle the stem and the seedlings may die. Often the lesions heal over as the plant ages. Rhizoctonia root canker also occasionally occurs on the upper tap roots of older plants as discrete, reddish brown lesions.

Thielaviopsis basicola causes a black root rot on young seedlings and older plants. The dark discoloration of roots and the presence of typical chlamydospores (visible with a microscope) are diagnostic of this pathogen.

COMMENTS ON THE DISEASES

The fungi involved occur commonly in soils. Most Pythium spp. are active during cool, wet weather, but P. aphanidermatum is favored by high temperatures. Seedling diseases caused by Pythium spp. are usually not severe unless beans are planted in cold soils or over watered. Rhizoctonia solani is favored by warm soil temperatures, but varies widely in temperature requirements. Often the population of R. solani is higher when the crop follows alfalfa or sugarbeets.

MANAGEMENT

Cultural Control
For blackeyes, planting when the average soil temperature is greater than 68°F (20°C) and in soil that is moist but not overly wet is the first line of defense. Irrigation following planting favors both Pythium and Rhizoctonia. Plant shallowly to minimize exposure of susceptible areas of the hypocotyl.

Organically Acceptable Methods
Cultural control is organically acceptable.

Treatment Decisions
Seed treatments may be helpful in fields with a history of seedling disease. Include a fungicide that is toxic to Pythium, and another that is toxic to Rhizoctoniasolani. There are no treatments effective against Thielaviopsis basicola.

Common name Amount to Use**
(trade name)  
When choosing a pesticide, consider information relating to environmental quality.
 
SEED TREATMENTS (Pythium spp.)
A. MEFENOXAM
  (Apron) TL 2–4 fl oz/100 lb seed
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
 
SEED TREATMENTS (Rhizoctonia solani)
A. PCNB
  (RTU-PCNB 24%) 3 fl oz/100 lb seed
  MODE OF ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)
 
B. CHLORONEB
  (NU-Flow D 30%) 7 fl oz/100 lb seed
  MODE OF ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)
 
 
**  Mix with sufficient water to obtain full coverage.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, for fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446
Diseases
R. M. Davis, Plant Pathology, UC Davis
Acknowledgment for contributions to Abiotic Disorders:
A. E. Hall, Botany and Plant Sciences, UC Riverside
Acknowledgment for contributions to virus sections in Diseases:
R. L. Gilbertson, Plant Pathology, UC Davis
Acknowledgment for co-authorship of Ascochyta Blight:
C. A. Frate, UC Cooperative Extension, Tulare Co.

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