GREEN HOUSE/NURSERY EDITION


Foes in High Tunnels

Watch for these pests and diseases
By Kathleen Hatt




Greenhouse photo by Deyan Georgiev/shutterstock.com

Insects and diseases that attack field crops can become an even greater problem within the confined and humid conditions of high tunnels and greenhouses. Be especially on the lookout for Botrytis cinerea, leaf mold and powdery mildew, all diseases that favor tomatoes in indoor growing spaces. "While it's not truly new, powdery mildew was more prevalent last year in the Northeast than I have ever seen before," says Becky Sideman, Ph.D., associate professor and extension specialist in sustainable horticulture at the University of New Hampshire. Also keep an eye out for pests such as the yellow-striped armyworm.

Ubiquitous Botrytis

A scourge of indoor tomato crops, B. cinerea, also known as gray mold, can infect almost all aboveground parts, including stem, leaf, petiole, peduncle and fruit. Botrytis infection often starts at a point of damage or on any decaying tissue, such as leaves in contact with infected soil, leaves wounded by handling, within leaf scars from de-leafing or on flowers.

Often seen on dying tissues, gray mold appears on young plants as dry, gray-brown, velvety spores covering stems or leaves. When gray mold spores become airborne, they can be dispersed by wind, overhead irrigation or sprays, tools (especially pruning shears and knives), other machinery and workers. Low light, plant stress or a shift in fruit load can trigger spore germination. Spores germinate and penetrate the plant surface within five to eight hours on moist plant surfaces at the optimum temperature of 59 to 68 degrees Fahrenheit. Within a few days, a new infection can produce visible symptoms and masses of spores, and multiple infections can be expected within the growing season. Airborne spores that infect soft tissues, pruning wounds and blossoms can produce an epidemic situation when moisture is present.



Gray mold (Botrytis cinerea) on grapes.
Photo by Joseph Smilanick, courtesy of USDA -ARS .

The best defense is a good offense

Because Botrytis survives in plant debris and in the soil, the most important step in preventing and managing it is to remove dead or dying tissue from plants and from the soil surface. Refuse must then be taken far away from the high tunnel or greenhouse and preferably burned. Do not add gray mold-infected matter to composting material.

Good sanitation practices and a thorough year-end cleanup, including disinfection of machinery and tools, are essential to preventing gray mold. Since high humidity (greater than 80 percent) is conducive to the development of Botrytis, leave adequate spacing between plants and consider increasing air circulation by removing lower leaves from overly shaded areas. Especially on nights when a significant temperature drop is expected, maintain adequate heat and ventilation. Good ventilation, both day and night, is critical to preventing and controlling gray mold.



Botrytis cinerea sporulation on a ripe strawberry.
Photo by Sc ott Bauer, courtesy of USDA -ARS.

To minimize the prolonged wetness on plant surfaces that is favorable to development of Botrytis, avoid overhead irrigation or spraying in late afternoon or on cloudy days. Spray or irrigate in the morning. Try to keep water from puddling on surfaces, including the ground or floor.

Fungicides or biological control agents should be rotated to prevent development of resistant populations.

Leaf mold lurks

Sometimes also a problem on field-grown tomatoes, leaf mold caused by Fulvia fulva is primarily a disease of indoor-grown tomatoes and is most serious in plastic high tunnels where relative humidity is high. Leaves are usually the only structure affected. Beginning as pale green or yellowish spots on the upper leaf surface, leaf mold lesions eventually coalesce, and an olive green mold develops on the lower leaf surface. Infected foliage curls, withers and may drop from the plant. On the fruit itself, a dark leathery rot develops at the stem ends.

As with Botrytis, leaf mold survives in plant debris and in the soil. Contaminated seed may also be the initial source of F. fulva. Leaf mold can be spread by rain, wind, tools and workers' clothing, as well as by insects. It is dependent on high relative humidity and high temperatures and generally does not occur where relative humidity is less than 85 percent.



Uncinula necator (or Erysiphe necator) causes powdery mildew in grapes.
By Macc heek at English Wikipedia (CC BY -SA 3.0), via Wikimedia Commons.

Again, good sanitation practices and a thorough year-end cleanup, including disinfection of all surfaces, machinery and tools, are essential. Remove and destroy all plant debris. Do not compost it.

Begin the new growing season with certified disease-free or hot-water-treated seed. The foliage of growing plants should be kept dry; use fans to circulate air. Removing bottom leaves and staking plants helps increase circulation and reduce the spread of leaf mold. Nighttime temperatures should be higher inside than outside. Rotate fungicides to avoid disease resistance.

Powdery mildew

Emerging as a significant disease of indoor tomato crops, powdery mildew is caused by several species of fungi and occurs in a large number of plant families, including Solanaceae and Cucurbitaceae. First appearing as light green to bright yellow lesions on the upper surface of leaves, powdery mildew spots then enlarge. On the undersides of leaves, a light powdery coating may appear. Then a dense white layer of growth may form on both leaf surfaces and entire leaves will wither and die, still attached to the stem. On tomatoes, the infection does not produce symptoms on fruit or stems.



Powdery mildew on a tomato plant.
Photo by Gerald Holmes, Valent USA Corp., Bugwood.org.

Unlike other fungal plant pathogens, powdery mildew does not need water to appear. It happily overwinters in greenhouses or high tunnels on weeds or plant debris, but cannot survive outdoors during winter in northern climates.

To prevent or control powdery mildew, control host weeds both inside and outside the greenhouse. Water carefully from below and use fans to improve air circulation. Pruning and staking help improve ventilation and light penetration. Several resistant varieties of tomatoes are now available, as are a number of organic and conventional fungicides.

Yellow-striped armyworm



Yellow-striped armyworm larva on a peanut plant.
Photo by Steve L. Brown, University of Georgia, Bugwood.org.

In December 2012, researchers at Pennsylvania State University reported that a new insect pest, yellow-striped armyworm (Spodoptera ornithogalli), had shown up on tomatoes in Pennsylvania the previous growing season. In New Hampshire, Sideman reports that the pest was found in several tomato tunnels that same season. Both yellow-striped armyworm and its relative, fall armyworm, a corn pest, have traditionally overwintered in southern areas and migrated northward each spring.

While commonly present in Pennsylvania, the number of yellow-striped armyworms had not previously reached pest status. Researchers postulate that the relatively warm winter of 2011-2012 enabled the pest to extend its winter range northward and that this is likely to continue as the climate changes due to increasing greenhouse gases in the atmosphere.

Yellow-striped armyworm larvae are distinct from other armyworms. They have a yellow stripe running the length of their body. Adults look like fall armyworms and are more difficult to distinguish.

Although yellow-striped armyworm is primarily a foliage feeder that can seriously injure young plants early in the growing season, it can also cause problems later in the season when it feeds on both foliage and the surface of the fruit. Since more mature stages of the pest have become tolerant of some insecticides, it is easier to control small larvae. For early detection and control, regularly monitor transplants and young plants. Within greenhouses and high tunnels, biological controls may be effective if used early. Conventional insecticides labeled for use in managing beet armyworm or fall armyworm should work against yellow-striped armyworm.

Once established, the problems mentioned are all difficult to manage. Fungicides as well as cultural tactics may be necessary to fight the diseases. Pests may require the use of an insecticide for control. Most important, however, is to "start clean, rotate and scout," says Cheryl Smith, Ph.D., extension professor and plant health specialist with University of New Hampshire Cooperative Extension.

Kathleen Hatt is a freelance writer and editor and a frequent contributor to Growing. She lives in Henniker, N.H.