What Are Root Disease Pathogens?

What are the different root diseases for greenhouse crops?

Fertilizer toxicity and pathogenic fungi are the two main causes of root problems. Toxicity from fertilizers can make roots more susceptible to fungi. Fertilizers can cause excessive soluble salt levels, suffocation, and nitrogen toxicity when excessive amounts accumulate around the roots of a plant. When soil is wet from irrigation and fertilizers, molds and fungi can thrive.

Symptoms of Root Diseases

1. Healthy roots are generally white and firm
2. Decayed roots may be watery, darkened, and easily crushed between the fingers
3. Some root fungi move into the stem and cause canker (aka blackleg)

Types of Root Disease Pathogens

Pythium is a species of microorganisms that can cause dampening off and stem rot. Another species that is related to Pythium is Phytophthora. Phytophthora is less common than Pythium but infects plants more aggressively and causes foliar blight in addition to rot. Both thrive when soil is very moist and excessive nitrogen levels are present. Pythium and Phytophthora are not true fungi (they are oomycetes) so not all fungicides are effective against them. Some pesticides indicated for them can also be used to treat true fungi like Rhizoctonia and Thielaviopsis.

Rhizoctonia is a common cause of root diseases and stem canker, the Rhizoctonia solani species can cause dampening off, rot, and web blight as well. Rhizoctonia is a true fungus and favors dry soils. Thielaviopsis causes a type of root and stem rot called black rot and can survive for years in contaminated soil. Thielaviopsis thrives in soil with an alkaline pH and can be inhibited by keeping soil pH near 5.5. Other common root pathogens include Fusarium, Sclerotinia, and Cylindrocladium. Laboratory tests are needed to determine the exact organism responsible for the symptoms.

Most fungi that cause root disease are a natural part of the soil and can survive indefinitely. Soil, tools, flats, and infected transplants introduce these pathogens into the greenhouse. Pests like fungus gnats and shore flies can introduce and spread root pathogens as well. Washing reusable tools and planters with soap and disinfecting in a 10% bleach solution help prevent the spread and introduction of pathogens. Soil should be decontaminated using steam (at 180°F for 30+ minutes) or fumigants.

Natural Pesticides for Greenhouse Root Pathogens

Using natural pesticides is a much better option than using chemical products that carry exposure risks. When considering pesticides (natural and chemical), be sure to remember that they need to be approved for greenhouse use and for use on your specific crop, they need to be stored according to a chemical hygiene plan, and you also need to have worker safety protocols in place.

Azadirachtin: Biodegradable neem seed extract effective against over 200 insects

Bacillus thuringiensis (BT): Beneficial BT soil bacteria effective against insects and nematodes

Neem Oil: Neem oil is less effective than azadirachtin, spray at sundown like with insecticidal soaps

Essential Oils: Tea tree, peppermint, clove, cinnamon, garlic, rosemary, and others can be effective against spider mites, thrips, aphids, fungus gnats, powdery mildew, botrytis, and more

Insecticidal Soap: Insecticidal soaps are potassium-based fatty acids effective against soft-bodied insects (aphids, spider mites, and whiteflies), should be sprayed at sundown and not used during flowering

Mineral Oil: Great for removal of pest eggs and effective against insects and fungi like powdery mildew

Potassium Bicarbonate: Salt mixed one teaspoon with 4 oz milk and 28 oz water is effective against fungus, especially powdery mildew, can be sprayed as a preventative treatment, or you can buy a ready to use product

Pyrethrins: Pyrethrins is for use in the vegetative growth stage, chrysanthemum plant-derived organic pesticide effective against flying insects like aphids, gnats, mites, thrips, and whiteflies

Fungicides for Pythium and Phytophthora

• Foestyl-Al (Aliette® WDG)
• Etridiazole (Truban® WP, EC and G)
• Dimethomorph (Stature)
• Fluopicolide (Adorn)
• Fludioxonil plus mefenoxam (Hurricane)
• Mefenoxam (Subdue® Maxx)
• Phosphonates (Alude, Fosphite, Vital)
• Propamocarb (Banol)
• Thiophanate methyl plus etridiazole (Banrot® 40WP)

Fungicides for Rhizoctonia

• Azoxystrobin (Heritage)
• Thiophanate methyl (Cleary's 3336, T-Storm, Allban Flo)
• Iprodione (Chipco 26019®, 26GT, Iprodione Pro)
• PCNB (Terraclor® 75WP)
• Pyraclostrobin plus boscalid (Pageant)
• Thiophanate-methyl plus chlorothalonil (Hurricane)
• Thiophanate-methyl plus etridiazole (Banrot® 40WP)
• Flutolanil (Contrast)
• Fludioxonil (Medallion)
• Trifloxystrobin (Compass)
• Triflumizole (Terraguard 50 W)