Fact Sheets

Pest Management

Root Diseases of Greenhouse Crops

Introduction
The two most common causes of root impairment of greenhouse crops are fertilizer toxicity and plant pathogenic fungi. In some cases, fertilizer toxicity predisposes plants to pathogenic fungi. An accurate diagnosis is necessary to manage the problem effectively.

Abiotic Causes
Abiotic (non-living) causes of root disease include excessive soluble salts, ammonium toxicity, and suffocation. Most commercially available fertilizers are in the form of salts. When excessive amounts of salts are in the soil solution, they desiccate plant roots. Ammonium toxicity may occur when fertilizers containing urea, or ammonium sulfate are used. Excessive levels of ammonium may also occur following steaming of organic soils, especially those containing manure. The conversion of ammonium to nitrate is carried out by soil microorganisms which are non-existent or in low numbers in soil-less media. The conversion can be inhibited by certain pesticides, cool wet soil, low pH, excessive soluble salts and poor aeration.

Roots must have oxygen or suffocation will occur. Soil composed of very fine particles is dense and has few air spaces. Similarly, a waterlogged soil contains little air. Plant pathogenic water molds thrive under saturated conditions.

Root Diseases Caused by Fungi
Extensive root decay will result in wilt but earlier symptoms can be seen by removing the pot and examining the roots. Healthy roots are generally white and firm; decayed roots may be water-soaked in appearance and/or darkened and easily macerated between the fingers. Some root colonizing fungi will move into the stem and cause canker or "black leg".

Pythium is one of the most common fungi found in the roots of greenhouse crops and is often associated with excessive nutrient levels or ammonium toxicity. The genus includes about 90 species which vary in their pathogenicity to plants and sensitivity to fungicides. Phytophthora, a related fungus, is generally more pathogenic than Pythium but is encountered less frequently. Pythium and Phytophthora are most destructive when soil moisture is abundant.

Rhizoctonia is also a common cause of root disease and stem canker. Unlike Pythium, dry soil is more favorable for disease development. For this reason, Rhizoctonia is more active in the upper portion of the soil. Other plant pathogens occasionally encountered include Thielaviopsis, Fusarium, Sclerotinia and Cylindrocladium. A laboratory diagnosis is necessary to determine the cause of root rot.

Sources of Root Disease Fungi
Fungi that attack root systems are natural inhabitants of the soil and thus, have the ability to survive there indefinitely. They are easily introduced into the growth medium by soiled hands, tools, flats and colonized transplants. Dirt floors may also harbor pathogenic fungi so it is important to keep the hose-ends off the floor. When a soil-less medium is ammended with field soil, it must be treated to prevent the introduction of plant pathogens, nematodes, insects and weeds. Fumigation or steaming of soil-less media is not recommended. However, when a soil-less medium becomes contaminated with plant pathogens, root rot can develop quickly.

Management
Pots or flats that have been used should be washed with soap and disinfested in 10% household bleach or a similar agent. If field soil is used wholly or as an amendment to a soil-less medium, it must be treated. Steam is the least expensive, safest and most effective method. The whole soil mass must reach a temperature of 180 F for at least 30 minutes.Various fumigants such as methyl bromide or Vapam® may also be used. Fumigants can be hazardous and must be handled cautiously. Residual fumigant in the treated medium may be phytotoxic. Be sure to follow the directions closely. For some crops, protectant fungicides should be used from the beginning of the planting cycle and repeated at regular intervals. Banrot® or other appropriate combinations of fungicides will provide a broader spectrum of activity.

Prepared by Dr. Robert L. Wick, Department of Plant Pathology, Fernald Hall, Room 111, University of Massachusetts Amherst, MA 01003. Tel. (413) 545-1045 Fax (413) 545-2532

updated 7/03

Fungicides for Pythium and Phytophthora

Common name	Trade name	Rate	Comments
fosethyl-Al	Aliette® WDG	0.4 to 0.8 lb/100 gal; 2 pts/sq ft.	Drench; however, foliar applications of 2.5-5 lb/100 gal will control root rot of some plants.
etridiazole	Truban® WP, EC and G	--------------------	Rates vary depending on the formulation.
metalaxyl	Subdue® 2E	0.5-2 fl oz/100 gal;	Rates vary depending on the plant. Subdue has broad crop clearance for ornamentals.
thiophanate methyl plus etridiazole	Banrot® 40WP	4-12 oz/100 gal; apply to 400 sq ft (about 0.5 pt/6" pot).	Irrigate immediately with additional water equal to at least half the volume of the fungicidal drench. Also controls Rhizoctonia, Fusarium, Thielaviopsis and Cylindrocladium.
propamocarb	Banol	20-30 fl oz/100 gal; see label for details.	Drench at 3 to 6 week intervals. May be tank mixed with thiophanate methyl for control of Rhizoctonia.
etridiazole	Terrazole 35 WP	3.5-10 oz/100 gal; drench with enough to saturate the soil such as 0.5 pt/6 inch pot.	Repeat at 4-12 week intervals.

Fungicides for Rhizoctonia and some other fungi

thiophanate methyl	Cleary's 3336	----------------------	Cleary's has broad crop clearance for ornamentals. Use experimentally for plants not on the label. Rates vary depending on the formulation.
iprodione	Chipco 26019®	6.5 oz/100 gal; 1-2 pts/sq ft.	Active primarily against Rhizoctonia.
PCNB	Terraclor® 75WP	4 oz/100 gal; apply to 800 sq ft.	Broad crop clearance for ornamentals. Active primarily against Rhizoctonia and Sclerotinia. May cause phytotoxicity to some foliage plants.
see above	Banrot® 40WP	see above	see above
fludioxonil	Medallion	1 to 2 oz packets/100 gal..	For Rhizoctonia, apply sufficent water to wet the top half of the growing medium. For other pathogens, completely drench the growing medium.
triflumizole	Terraguard 50 W	4-8 oz/100 gal; 4 fl oz/6 inch pot.	For best results do not irrigate with additional water until 24 hr after application. Apply at 3-4 week intervals as needed. Do not use on impatiens plugs. On impatiens transplants, do not exceed 2 oz/100 gal.