Preventing Late-Season Root Rot – Thielaviopsis

Many of the most profitable crops are highly susceptible to root rot caused by Thielaviopsis basicola. Diseased roots impede the uptake of nutrients and water, resulting in poor and uneven growth. However, you can protect the roots of your crops as well as your margins with preventive management programs.

Thielaviopsis basicola is the causal agent for this disease, which leads to roots on affected plants to become speckled with light brown lesions. In some crops, the disease causes dark, sunken lesions or cracks on the lower stem.

Be on the lookout for the most common symptoms:

  • Yellowish/pale green leaves with green veins
  • Bottom leaves curled downward
  • Dark, blackened roots
  • Sunken lower stem lesions 
  • Stunted plants

Diseased roots cannot absorb nutrients and water. These infected plants are weakened and exhibit signs of stunted growth and nutrient deficiency. When bedding plants become infected, their uneven growth can make an entire flat unmarketable even though not all plants are diseased. Replacing stunted plants in the flat is time consuming and costly. Severely diseased plants often die.

Cultural Tips

A dry, clean growing environment coupled with preventive fungicide treatments can protect susceptible crops.

  • Identify the cause of root rot. Above-ground symptoms can mimic nutrient deficiencies and other root rots caused by Pythium, Rhizoctonia and Phytophthora
  • Keep the production area clean. Thick-walled spores survive in soil/growing media, wooden benches, growing mats and plant containers.
  • Maintain a slightly acidic growing medium. Neutral or alkaline soils favor disease. • Limit the spread. Remove infected plants/plant parts immediately and place infected plants into closed containers.
  • Maintain optimal growing media conditions. Prolonged periods of wetness or saturated media conditions favor disease development.

Preventive Measures

Apply fungicides prior to disease when the crop is susceptible or if the greenhouse has a history of Thielaviopsis. Drench every 21 days with fungicides using labeled rates and alternate among the following:

  • Medallion® WDG fungicide: 1 oz/100 gal
  • Terraguard® fungicide: 4 oz/100 gal
  • Thiophanate methyl: see labels for rates
For active infections, use high labeled rates of fungicides, applying every 14 days and alternating:
  • Medallion WDG: 2 oz/100 gal
  • Terraguard: 4–6 oz/100 gal
  • Thiophanate methyl: See labels for rates
In addition to Thielaviopsis, Medallion WDG helps control other foliar, crown, stem and root diseases such as Fusarium, Rhizoctonia, Cylindrocladium and Sclerotium rolfsii. Its water-dispersible granule formulation means it is easier to mix, measure, store and apply. Medallion WDG is an ideal choice for late-season root rot as it binds quickly to the surface of leaves and roots, creating a barrier of protection.

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©2018 Syngenta. Important: Always read and follow label instructions. Some products may not be registered for sale or use in all states or counties and/or may have state-specific use requirements. Please check with your local extension service to ensure registration and proper use. GreenCast®, Medallion® and the Syngenta logo are trademarks of a Syngenta Group Company. Terraguard® is a trademark of OHP, Inc.

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