As the chill enters the air and the outdoor growing season for those in cold weather areas comes to a halt, high tunnels offer vegetable growers a chance at season extension. But keeping greens under cover doesn’t mean that pests and diseases aren’t going to be a problem.

Take spinach. According to Jim Correll, University of Arkansas, speaking during a University of Rhode Island Winter Spinach Field Day webinar, spinach production in the U.S. has grown enormously in the past several decades, leading to many disease pressures, including downy mildew – arguably the most serious.

Spinach has a short time to maturity (often 25 days) and large-scale field production exists primarily on the West Coast. High tunnel production has grown as well in cold weather regions, where the crop is hand-harvested and it can benefit from cold weather, developing more flavor, Correll said.

With the prevalence of spinach being grown both in the field and in high tunnels, spinach downy mildew has proliferated in recent years.

Diseases in High Tunnel Spinach

Growers need to be aware of several other common high tunnel diseases, including soilborne disease, especially when cold, wet and cloudy weather patterns are present. Soilborne fungal diseases – about four or five different pathogens – exist in all soils and attack seedlings before or soon after they germinate, Ann Hazelrigg, University of Vermont Extension plant pathologist, said in webinar for the Vermont Vegetable and Berry Growers Association.

“All of these fungi like cool, wet conditions. Don’t let those seeds sit in cold, wet soil,” she said. “Anything you can do to promote rapid germination will help.”

Disinfecting flats prior to seeding, using heat mats to speed germination and not overwatering are best practices. Soils high in organic matter will hold more moisture and effect the amount of watering needed.

The use of a beneficial Trichoderma fungus, which competes with pathogens for root sites on plants, affords protection from damping-off concerns.

Disease concerns in high tunnel spinach include leaf spot disease caused by the pathogen Cladosporium variabile. The discrete, small spots can resemble those caused by abiotic issues, but fungal spores will form after a few days. The spores can live for eight years in dead spinach tissue.

Rotating out of spinach each year, as the disease will build up over time, is warranted. Avoid the use of row covers if plants are wet, and utilize chlorine or hot water treatment of seeds, to keep high tunnels free of the pathogen. There are resistant cultivars available.

Cladosporium can thrive in temperatures ranging from 41º – 86º F, although it prefers cool, moist conditions between 59º and 68º. Relative humidity of over 80% promotes disease growth and spread.

Drip irrigation can keep leaves dry. Monitoring soil moisture can be tricky, however, particularly when daytime temperatures fluctuate from cold to warm and when day-to-day conditions can vary.

Edema is an abiotic disease which can occur when plants take up water, but conditions don’t allow for it to transpire. Plant cells will rupture, causing lesions on the undersides of leaves. Plant will recover as they grow. It’s common in high tunnels due to the difficulty of watering during temperature fluctuations and changing weather conditions which occur during the cold weather months.

Grandular trichomes occur naturally on spinach, and are not a disease, but are more prevalent on spinach grown in high tunnels. These white, droplet-like epidermal outgrowths store metabolites and protect the plant from stressors.

Powdery mildews like high humidity, but with warm and dry conditions. A white coating on upper or undersurfaces of leaves are spores, which are windblown and can overwinter. Powdery mildews are very host specific, but they all like the same environmental conditions. Powdery mildew typically occurs late in the season. Crops should be destroyed, as the spores will travel and spread the disease.

Downy mildew is an increasing prevalent problem in spinach, Hazelrigg said. It appears as yellow splotches on the tops of leaves, and corresponding brownish-purple spots will develop on the undersides. These spots may be most visible in the morning, as they are produced overnight, or under conditions of high humidity. Spores are released and spread via air.

There are resistant strains of spinach. However, there are at least 19 known races of downy mildew, and one new race per year is typical. The fungus-like pathogen (it’s considered an oomycete or water mold) prefers moist and humid, wet conditions. Peronospora farinosa f. sp. spinaciae is the causal pathogen. Resistant cultivars are recommended, particularly to races known to be prevalent in the growing region, although the pathogen can travel long distances.

Spinach has separate male and female plants. Combining the genetics of cultivars for downy mildew resistance in hybrid production is an important part of combating the disease, Correll said. Combining male and female plants from different cultivars creates hybrid plants with enhanced resistance to a wider range of downy mildew strains. But novel strains are continuing to evolve. To complicate matters, female spinach plants can produce pollen and fertilize themselves at times, creating plants that are not hybrid and therefore more susceptible to downy mildew races, and can allow the pathogen a foothold even when hybrid cultivars are planted.

Fungicides can be used preventatively. In certified organic systems, copper is recommended. Decreasing humidity and moisture are important control measures, as is rotating out of spinach for at least two or three years.

The asexual sporangia need a live host to survive, but can survive up to two weeks without one. These spores can blow in the wind, traveling long distances. Favorable environmental conditions will then allow the pathogen to infect its next host. Downy mildew pathogens are not able to infect other crops, as the pathogen is host-specific. Some retail spinach has been found to have sporangia on it.

Dormant oospores (sexual structures) are also present, but they are not well understood. They can be found on seed and in soils and cause infection.

Downy mildew can spread rapidly once environmental conditions are right. It prefers wet leaf surfaces and temperatures between 40º and 80º.

“It can really spread fast. It can be sort of an epidemic. You can go from spore to infection to producing another spore in less than seven days,” Hazelrigg said.

The “green bridge” factors into the prevalence of the disease, Hazelrigg said, as spores from field-grown spinach can travel into high tunnels, spreading the disease.

“Anything you can do to break that green bridge, like tilling under your fall spinach, so you get a break before you plant the high tunnel spinach, is a good idea,” she said.

Weed control both inside and outside the high tunnel is important, as many disease pathogens can overwinter in weeds. Aphid-vectored diseases such as cucumber mosaic virus of spinach are also of concern, and biocontrols of aphids can significantly reduce disease pressures.

Spinach is a high-value crop, with the potential for high tunnel growers in cold climate regions to capitalize on its popularity, if they can overcome some of the primary disease concerns seen with the crop.

“There’s always challenges in growing spinach,” Correll said.