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In High Tunnels, Diseases Are What You Make Them

Thomas A. Zitter|
Professor
Department of Plant Pathology
Cornell University
Ithaca, NY 14853

taz1@cornell.edu

Introduction

            Disease control measures taken in high tunnel structures are not that much different from those undertaken in greenhouse environments, but there are some important differences. The environment is not as stable as that found in greenhouses and requires more daily time inputs and more planning when the facility is constructed. Daily raising and lowering of the plastic sides each morning and evening will provide the necessary control of temperature and humidity. High tunnel orientation is critical to ensure movement of prevailing winds through the tunnel sides. Since high tunnel units should be perceived as semi-permanent, they should be situated on the best well-drained soil and with the appropriate pH and nutrient adjustments made prior to crop establishment. Use of raised, plastic mulch covered beds with drip irrigation tape buried beneath each bed will afford soil warming and more precise soil moisture control. The eight sustainable plant protection practices given below, when used in concert with adequate planning, will provide the desired level of disease control.

Sustainable Control Practices

            1. Best Crop Growth Practices:  Maintaining optimum crop growth by providing adequate nutrients and soil moisture will allow plants to grow without interrupted growth that otherwise could favor disease development. Do not provide over-rich soils that lend themselves to rank plant growth and likely bacterial infections as is the case with tomato pith necrosis infection of tomato by Pseudomonas corrugate (Fig. 1).

            2. Manipulation of the Environment:  Temperature and humidity are the two main factors that must be considered. The length of time above 90% relative humidity should be limited as high humidity favors many disease pathogens. Although leaf mold of tomato (Fulvia fulva syn. Cladosporium fulvum) does not occur every season, high relative humidity is particularly favorable for disease development (Fig. 2). These conditions are also appropriate for Botrytis gray mold (Botrytis cinerea) (Fig. 3). The oomycete pathogen causing late blight of tomato leaves and fruit (Phytophthora infestans) requires high moisture and cool to moderate temperatures for infection to occur (Fig. 4 a, b).

Fig.1 Pith necrosis Fig. 2 Leaf mold Fig. 3 Gray mold  Fig. 4 a, b. Late blight on leaf and on fruit

The operator needs to provide high tunnel ventilation as early and as late in the day as possible by raising and lowering the plastic sides. It is also necessary to provide adequate drainage around the base of the structure to make sure no water flows or seeps into the high tunnel. Wet soils promote the development of soilborne and other foliar diseases.

            3. Disease Resistant Varieties:  Always choose disease-resistant varieties whenever possible. Also select varieties that tolerate diseases especially in high-tunnel culture.

            4. Disease-free Seed and Transplants:  Make sure to start with disease-free seed or transplants. Seed can carry bacterial, fungal and viral pathogens, so seed needs to be purchased from reputable dealers, or should be treated by the grower before seeding. Be especially mindful of tomato and pepper seed that carry bacterial speck, spot and canker (Fig. 5). The same care should be exercised in the growing of healthy transplants. Bacteria can survive in the soil and on wooden surfaces that become contaminated.

            5. Crop Rotation:  If several high tunnel units are to be used, then rotation of crops between structures is possible. If soilborne pathogens reach an unacceptable level, it may be necessary to consider soil fumigants or even grafting plants onto resistant rootstock. Fusarium wilt (F. oxysporum f. sp. radicis-lycopersici) can be particularly devastating (Fig. 6). Grafting is commonly employed for solanaceous and cucurbitaceous crops in many areas where susceptible crops are grown in the same facility on an annual basis (see Fig. 12).

            6. Sanitation In and Around the Structure:  Remove and destroy any disease tissue as soon as it is found. Do not allow workers to move freely between high tunnels and field production areas with contaminated clothes and foot ware. When the crop is finished, remove and destroy all plant material including the roots. Do not allow any volunteer plants to become established in the structure at any time. Volunteer weeds can be a source of insects like aphids and thrips that can also serve as vectors of virus diseases. Maintain a crop and weed-free area around all permanent structures to reduce the movement of insects and pathogens into the sheltered area (Fig. 7). Insects such as the tarnish plant bug are attracted to tomato when alfalfa is harvested or weeds are mowed, and their injury allows entry of Alternaria alternata into wounds (Fig. 8).

            7. Avoid intermixing vegetables and potted ornamentals in the same tunnel. Tomato spotted wilt virus (TSWV) vectored by thrips is common on ornamentals and are vectored by the immature thrips to crops like tomato (Fig. 9).

            8. Soil Fumigants and Fungicides as Needed:  Use of rotation, sanitation, and clean plant materials should minimize the need for soil fumigants and foliar sprays. Still it is not always possible to keep all pathogens out of the structure, and some use of fungicides may be necessary.

Fig. 5. Bacterial canker control requires sanitation on any wooden surface; leaf marginal infection; stem infection; fruit infection.

Fig. 6. Fusarium wilt   Fig. 7 Weeds encourage insects Fig. 8. Tarnish plant bug Fig. 9 Tomato spotted wilt virus

Potential Disease Problems and Their Control

Powdery Mildew:  Powdery mildew of cucurbits caused by Podosphaera xanthii (formerly Sphaerotheca fuliginea) and Erysiphe cichoracearum can be expected if cucurbits are grown in tunnels for any extended period of time (Fig. 10a). Mildew produces white powdery colonies on upper and lower leaf surfaces, on stems and petioles, and colonies are visible with the naked eye (Fig. 10b). Powdery mildew development is favored by dry plant surfaces.  In high tunnel production, the time that plant surfaces remain wet is considerably shorter than for plants grown in open field culture. Although nighttime dew formation is favored out-of-doors, in tunnels the plastic covering slows the amount of radiation cooling of the plant surfaces, lessening dew formation, and creating an ideal environment for powdery mildew to develop. Powdery mildew of tomatoes (caused by Oidium neolycopersici) is also commonly seen in high tunnel facilities, and may infect other solanaceous crops and weeds and other crop species (Fig. 11). Infected rosemary has been implicated as the source of powdery mildew in tomatoes. Contact fungicides do a reasonable job of controlling mildew if an effort is made to cover all plant surfaces, applying them on a 5-day schedule, and beginning application as soon as infection is found. Mineral oils (JMS Stylet Oil) and potassium bicarbonate (Armicarb, MilStop, and Kaligreen) have been used. Systemic fungicides such as the strobilurins (Quadris and Flint) and the demethylation inhibitor Nova can be effective tools for mildew control. Because these are site-specific fungicides, they are prone to the development of resistance and should be alternated with the protectant materials given above, and if they prove ineffective after application, any further use should be discontinued.

            Foliar Blights:  If proper steps have been taken to reduce humidity and moisture condensation, the number of foliar diseases that occur should be minimal, including common early blight of tomato. Late blight has been reported in tunnels if high humidity exists (Fig. 4a, b). If late blight should occur, immediately remove any infected tissue and allow the house to dry out. Quadris performs quite will on late blight as does Phostrol mixed with mancozeb. Products should be alternated and applied on a 5-day schedule until the disease has been eradicated.

            Soil Borne Diseases:  These diseases should not be an issue if protective steps of rotation and sanitation have been followed. Soil fumigants such as Vapam HL or K-Pam HL are multipurpose soil fumigants that provide both disease and nematode control. Grafting onto resistant rootstock is also practiced for solanaceous and cucurbitaceous crops (Fig. 12).

            Virus Diseases:  Use of virus resistant varieties and virus-free seed should reduce the chance for viruses like tobacco mosaic virus in tomato and pepper. Clean sanitation around tunnels of common weeds like common chickweed, milkweed, spiny sowthistle, dandelion, and blackseed plantain will eliminate the buildup of insects, and the chance spread of viruses like cucumber mosaic (CMV) and tomato spotted wilt (TSWV) (Fig. 9).

 

Fig. 10 a, b Powdery mildew of cucurbits    Fig. 11 Powdery mildew of tomato   Fig. 12 Grafting

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