For many growers, winter feels like the time to retreat indoors and focus on seed orders or spreadsheets. However, plant pathogens do not simply vanish when the temperature drops; they either hunker down in your field waiting for spring or actively thrive in the cool, humid environment of your high tunnel. Therefore, a "winter walk" can save you time and money to get ready for incoming spring.

If you are growing winter greens, spinach, or overwintering tomatoes, your high tunnel is essentially a "cool, damp box." It provides the optimum environment for specific fungal pathogens that favor high humidity and lack of airflow.

1. Gray Mold (Botrytis cinerea)

   This is the number one winter threat. It thrives in temps between 55-75°F.

   • What to look for: A fuzzy, gray-brown spore mass forming on wilting and dead plant tissues, pruning wounds, or lower leaves touching the soil.
   • Ghost spots of tomatoes: Tiny pale white halos or ring spots on the fruit. This symptom indicates an infection that didn't rot the fruit but will ruin its marketability.
   • Action: Sanitation is key. Remove all diseased and dead plant debris immediately. If you see active sporulation, increase venting to lower humidity.

2. White Mold (Sclerotinia sclerotiorum)

   Also known as Timber Rot, this pathogen is deceptive because it often kills the plant from the inside out.

   • What to look for: Bleached, bone-white lesions at the base of stems of Solanaceae family plants such as peppers and tomatoes.
   • The signs: If you split a white, infected stem open, you will often find hard, black irregular structures that look like mouse droppings. These are sclerotia, the survival structures of the fungus.
   • Action: Remove the entire plant and the surrounding soil immediately. Do not put this in your compost pile.

3. The Mildews & Leaf Molds

   Winter greens are susceptible to foliar diseases that hide on the underside of leaves.

   • Spinach Downy Mildew: Look for yellow angular patches on the top of the leaf. Flip it over to see a purplish gray "dirty" fuzz.
   • Tomato Leaf Mold: Look for bright yellow blotches on the upper leaf surface. Underneath, you will find an olive-green, velvety mold.
   • Action: Scout the "warm spots" of your tunnel first (near heaters or the sunniest wall), as populations often explode there first.

Diagnostic Notes:

1. The "Purple Leaf" Imposter

   Don't panic if your tomato or spinach leaves turn deep purple. In winter, cold soil temperatures often make phosphorus unavailable to the plant roots, even if the nutrient is present in the soil. This is usually a physiological issue, not a disease. As soils warm up, the green color typically returns.

2. The "Bumpy Leaf" Mystery: Oedema

   When days are cloudy and tunnel ventilation is closed to trap heat, you might spot strange, wart-like bumps on your plants. Before you reach for a fungicide, check if it is Oedema, which is a physiological disorder, not a disease. It happens when the soil is warm and moist but the air is cool and humid. The plant takes up water faster than it can release it, causing leaf cells to rupture and form blisters. It is extremely common in winter kale, cabbage, spinach, and sometimes tomatoes. Therefore, don't diagnose it as bacterial disease or mite damage and spray. Watering wisely and increasing the air circulation will help.

Summary Checklist

To get a head start on the season, put these items on your to-do list for this week:

1. Ventilate: If your high tunnel has condensation dripping, it is too wet. Open the vents.
2. Sanitize: Remove dead leaves and "mummy" fruits from all growing areas.
3. Inspect: Use a hand lens to check the undersides of leaves in the tunnel and the bark crevices in the orchard.
4. Differentiate: Learn to tell the difference between cold stress (purpling) and active disease.

Winter scouting is not about finding problems to worry about; it is about finding problems you can solve before the busy season begins.