Fall High Tunnel Cucumbers: A Limited Window Opportunity

High tunnels provide the ideal environment to produce high quality cucumbers using trellises as harvests of field-grown cucumbers are dwindling. The window to turn a high tunnel around and replant with cucumbers is a bit narrow, but manageable.

For those unfamiliar with indoor cucumber production, one important difference is variety selection. Greenhouse and tunnel cucumbers should be parthenocarpic types. Parthenocarpic varieties can set fruit without pollination or fertilization of the ovule. When seeds are present, they are sterile. Commonly grown tunnel varieties include Corinto and Lisboa. In cucumbers, parthenocarpic varieties should be protected from pollinators or fruit quality can suffer. The most common symptom when parthenocarpic varieties are pollinated is misshapen or curved fruit. Plain window screening, used on the roll up sides, doors and vents not only prevents pollinators from entering, but also provides control of cucumber beetles. Unfortunately, smaller insects such as thrips, aphids, and mites can still enter the structure and will need to be managed.

If you grow a spring cucumber crop in your tunnel, harvests and quality are mostly dwindling by the end of June, and by then, field production will be pulling prices downward. What a great time to clean that old crop out and prep for a fresh crop that begins harvests in late August to early September. While there is some variation by variety, most parthenocarpic cucumbers will start producing fruit at about 45 days after the first true leaf emerges and can continue to produce fruit for another 45-60 days. If you replant via direct seeding to soil or into potting media between mid-July and August 1, you will begin harvesting from fresh, young vines by early to mid-September. It's important to get new plants started as early in this window as possible so they put on enough growth to 'coast' into the shorter days of September and October. Unless you provide artificial lighting, the reduction in growth is significant by early October.

Fall-Grown Cucumber Pest Management

Pest management of fall-grown cucumbers is in many ways reversed compared to spring cucumber production. Instead of a few weeks of low-level insect, mite, and disease pressure like we experience with early spring cucumbers, the fall crop is emerging into what is often a very high-pressure environment. Proactive management of likely insects, mites, and diseases is highly recommended!

Powdery mildew (PM)

Summer is PM season. As humidity increases and plants age, the likelihood of PM increases rapidly. Plants that are fruiting are much more susceptible to PM than young, vegetative plants. A proactive preventative fungicide program is necessary due to PM. PM looks like talcum powder and first appears as pencil eraser-sized spots that spread rapidly. Begin management of PM at emergence!

Downy mildew (DM)

DM is interesting as this disease does not overwinter in our area but does come in annually on storms from the south. Pay careful attention to the DM IPM PIPE program and begin management when DM is nearby. In cucumbers, yellow spots visible on the top of the leaf are usually the first signs that this disease has arrived. Rapid and timely treatment with a fungicide labeled for DM at the first signs of infection is necessary to avoid premature defoliation of your plants. Infected areas will continue to senesce (or die) even with treatment. Proactive treatment when nearby DM sentinel sites or scouting indicates disease presence can prevent infection.

Gray Mold / Botrytis cinerea

As the weather cools into late September, gray mold potential increases. High humidity and rainy conditions are ideal for Botrytis to rapidly infect new tissue. Flowers and young fruit are especially prone to infection. The best way to avoid Botrytis is through improving airflow and ventilation, but if it's rainy, then ventilation accomplishes very little. 'Peroxide' products like JetAg 5®, ZeroTol®, and Oxidate®, applied at maintenance rates during periods of high infection, can help to reduce inoculum. At the low concentrations used, it is safe and labeled to allow for daily applications during multiple day rain events.

Soil disease management

Fall cropping typically means going into soil or potting media that’s coming out of another crop, so the opportunity for soil disease inoculum is high. One method to reduce inoculum is to drench your soil with a 'Peroxide' product like JetAg 5, ZeroTol® or TerraClean following the label instructions. The 'peroxide' drench will significantly reduce disease inoculum, then follow up after at least 4 hours, but not more than 24 hours, with a re-inoculation with Actinovate® AG, RootShield® Plus, or your favorite microbial product.

Spider Mites

High tunnel crops get spider mites and summer tunnels are ideal for them. This is a simple fact of growing in a tunnel and cucumbers are no different than any other tunnel crop. The dry leaf conditions coupled with a well-irrigated and fertilized crop are ideal conditions for spider mites. Scout regularly, apply miticides as necessary.

Aphids and Western Flower Thrips

Both are common tunnel residents and are typically rapid colonizers of a new summer tunnel crop. Begin a proactive pest management program from emergence.

Striped cucumber beetles (CB)

Cucumbers are very susceptible to bacterial wilt vectored by cucumber beetles. There is some varietal difference in tolerance to bacterial wilt, but not enough to use as a management tool. The striped cucumber beetle has always been our dominant type, but it is not unusual to find spotted cucumber beetles by late summer. Plants die rapidly once fed upon by infected cucumber beetles. This is especially so for younger plants, so control is necessary from plant emergence through the end of harvest. There are several choices for insecticides in controlling CB populations. Rotate mode of action (MOA) often using the IRAC code, and include an aphid management program if using broad-spectrum insecticides. The screening noted above is worthy of consideration as it is inexpensive, quick, and easy to install and is highly effective.

Marketing

Markets will likely prefer varieties like Corinto and Lisboa due to their traditional appearance. Excelsior is the one pickling type that has performed well and can readily fill that niche (and it's super crispy), but your sale price may be an issue due to modest plant yields as compared to the price that consumers are willing to pay for pickling-type cucumbers.

Comments:

• To reduce input costs, reuse potting media from the spring production run in the fall. Strip the old plants and roots from the pots, then top off with fresh media and direct seed. Unless soil-borne diseases are present, there is no reason that a high coir/composted bark potting media could not last 2+ years (4 crop cycles). A wetting agent will likely be needed after the pots dry out completely between cropping cycles to get them to evenly wet again. Always use a soil inoculant such as Actinovate AG, RootShield Plus (or other similar product) to reduce the opportunities for pathogens to take hold.
• Direct seeding results in very compact plants that are tougher in the seedling stage than transplanted plants. Even using the larger spacing from 32 or 50 cell trays produces some stretching prior to transplanting. Direct seeding makes it easier to install the first trellis clip as the young vines are stronger than transplants. Cooler conditions in the spring favor transplants.
• Plant spacing: Practices vary a little by region: most PA growers use 2 plants per foot, NY growers tend to grow 1 plant per foot, and MN growers often use 3 plants per foot but always use a 'V' trellis with plants alternating direction on the 'V'.
• Cucumbers are grown with fertility similar to tomatoes. A 20-20-20 fertilizer (or similar) is used until plants start to flower and then switch to a high K fertilizer (9-15-30 or similar).

As you get through September and into October, Botrytis-related problems increase, and production of #1 fruit will rapidly decrease. Keep an eye on your costs and returns to determine when to shut things down and clean the house in preparation for the next spring run.