Molds are a large and taxonomically diverse group of fungal species. While different molds develop under mostly similar conditions, they harm crops in vastly different ways. From visual damage to complete plant death, molds are a serious problem in greenhouse crops. Once infected, it is not easy to eradicate these fungi from the greenhouse, so the best way to treat mold is prevention.

Here are some of the most common and widely distributed molds in greenhouse production. These diseases can cause significant economic losses in floriculture, leafy greens, and vegetable crops.

Powdery mildew

Optimal conditions: Day temperatures in the 20-30°C range and high night humidity induce mildew formation.

Symptoms: Whitish, talcum-like, powdery fungal growth develops on both sides of the leaf, petioles, stem, and fruit. Host tissue becomes chlorotic.

Botrytis cinerea

Optimal conditions: 20°C for germination and 24°C for growth. Relative humidity 85% or greater.

Symptoms: Brown discoloration of leaves, blighting of bracts and leaves. Buds turn brown and can develop a fuzzy grey growth.

Septoria

Optimal conditions: 20-25°C and high humidity.

Symptoms: Formation of leaf spots, circular to irregular in shape, usually develop from the base of the plant and merge upward. Spots can appear as black, brown, or occasionally reddish.

Rusts

Optimal conditions: 15-20°C and at least six hours of free moisture on the leaf surface.

Symptoms: Usually appears as yellow, orange, red, rust, powdery pustules on soft tissue such as leaves, young shoots, and fruits. Plant growth and productivity are commonly reduced, and some plants wilt and die.

The controled conditions and enclosed environment of a greenhouse are meant to support optimal plant growth. However, compared to open-air environments, all biological processes occur faster—whether positive or negative. Flora and fauna accumulate more intensively than in natural habitats. The optimal conditions that maximize plant growth and yield are also the optimal conditions for mold development. Temperature, humidity, lighting, and air circulation are key factors in the presence of mold infections. Finding the right balance between best conditions for crops and best conditions for molds is an ongoing struggle.

“Finding the right balance between best conditions for crops and best conditions for molds is an ongoing struggle.”

How to prevent and manage mold infections

• Increased ventilation is an important tool for disease control and can be achieved in several ways: Defoliation of old\unnecessary leaves can improve air movement. In addition, fans may be useful to promote air circulation.
• Avoid overhead irrigation. Use drip tape to reduce moisture on leaves and splash dispersal of pathogens. Wide row spacing will encourage the soil surface to dry out fast and increase ventilation.
• Minimize pathogen spread by using sterile equipment when moving between rows, greenhouses, or working on different crops. Make sure that the workers disinfect their hands before entering the greenhouse and during work.
• Scout for disease routinely. Early detection is critical for disease control.
• Sanitation is key. Physically remove all infected tissue or diseased plants that can serve as inoculum.
• Apply fungicides when greenhouse conditions are favorable for disease development.
• Crop rotation can reduce disease incidence. Many plant pathogens survive from one growing season to the next in the soil or on crop residues. Crop rotation with alternative non-host crops provides time to reduce pathogen populations.
• Use resistant varieties when possible.