Summer Fungi Aren’t So Fun

Summer weather has begun! While heat and humidity are favorable to growth for many of our favorite summer fruits and vegetables, they are also favorable to many diseases in the garden. The first signs – yellowing leaves, brown streaks, and white fuzzy patches – are easy to ignore at first, but left alone, they will spread throughout the plant and the rest of your garden fairly quickly, since the fungal spores are airborne. Here are some of the issues we’re seeing now, or will see soon:

Cucumbers, Zucchini, Squash and Melons – Powdery Mildew

Powdery mildew starts off as small, fuzzy white dots on the tops and bottoms of leaves of plants in the cucurbit family. Unless you’re looking at the plant very closely, it’s easy to miss, as the white dots can blend in with the textured leaves. Once the fungus has spread, the entire leaf will be covered with white or grayish-white ‘film’.

Powdery mildew on the surface of a cucumber leaf.

The first few spots of powdery mildew on the surface of a cucumber leaf.

Powdery mildew on the back of a cucumber leaf.

Powdery mildew on the back of a cucumber leaf.

As soon as you see this on your plant, carefully remove heavily infected leaves (leaves fully covered  with the white, fuzzy mycelium of the fungus) and deposit in the trash. Try to snip, rather than tear off the leaves, and be sure not to shake the powdery “dust” which contains the spores onto other leaves. Do not compost the leaves either, as the spores can survive until next year and go on to start new fungal infections. Be sure to remove leaves conservatively – losing too many will also set the plant’s growth back.

Treat leaves that have just a few spots, or leaves with no spots but are on the same plant as other infected leaves with any of the following sprays approved for organic gardening: GreenCure, Serenade, or Actinovate. Follow the directions on the labels carefully. Anti-fungal sprays are best applied on the tops and bottoms of the leaves in the evening, out of direct sunlight. You can also try a homemade anti-fungal spray by following this recipe: to one gallon of water add: 1 Tbs. baking soda, 1 Tbs. neem or vegetable oil, 1-2 drops of dish soap, but this is usually not as effective as any of the brands listed above.

The main thing about any spray is that it must be applied continuously throughout the season to be effective – and remember, this is a treatment to just prolong the life of your plant and keep the fungus in check. None of the sprays are a “cure”. Usually, a weekly spray schedule (more often if it rains) will be sufficient.

If you decide not to remove or treat the fungus, your cucurbit will experience much lower production and premature death, while propagating spores that may infect your neighbor’s plants.

Basil – Fusarium Wilt and Downy Mildew

Fusarium wilt first presents itself as brown splotches or streaks on basil stems and leaves, particularly along the leaf edges. These splotches are irregularly shaped and not usually round. After the fungus progresses throughout the plant for a few weeks, the basil will suddenly wilt and die. At this point, it may seem like your basil just needs to be watered, but watering will not fully revive it. This is because the fungus “clogs” the plant’s vascular system, inhibiting the uptake of water from the roots.

Fusarium wilt on a basil leaf. Brown splotches and streaks along the stem are typical.

Fusarium wilt on a basil leaf. Brown splotches and streaks along the stem are typical.

If the lower leaves of basil are yellowing, and others have a slight ‘bronze’ tone, check under the leaves for the fuzzy grayish-purple spores of downy mildew, shown below:

Downy mildew spores on the back of a basil leaf.

Downy mildew spores on the back of a basil leaf.

Fusarium wilt and downy mildew on basil can be treated with the same products, and with the same application as powdery mildew.

Why do these fungal diseases hit? Powdery mildew, downy mildew and fusarium all thrive in warm weather. They are pervasive in commercial greenhouses, and can even come on the seeds themselves. Fungal diseases, like many plant pests and pathogens, tend to infect weakened plants first. Plants that are stressed (inadequate nutrients, extreme temperatures or temperature fluctuations, too much water or too little water) essentially have fewer resources to spend on “defense”.

What can I do to prevent this? You can help keep your plants from being stressed by first applying an organic granular fertilizer in the spring at planting time, and again around the beginning of summer. Fertilizers that contain added beneficial bacteria and fungi are even better – when they are colonizing the surface of the plant’s roots and leaves, there is less space for a pathogenic fungus to gain footing.

Although you can’t control temperature fluctuations, you can ease the stress on your plants by applying a mulch such as straw over the surface of the soil. This will help keep the surface from drying out or heating up too quickly. Straw can be hard to find, but dead leaves will also work. If you have weeds in your garden, you can use these as a quick mulch as well. Make sure they don’t have seeds in the flower heads and cut off the roots before laying on the soil surface.

Many of the ‘treatments’ sold for fungal diseases actually work better as preventative measures. Products such as GreenCure and Actinovate (for air-borne fungal diseases on a variety of plant leaves), and Rootshield (for soil-borne tomato fungal diseases first affecting the roots and stems) are best applied before you have a problem.

You can also prevent diseases by choosing varieties that have documented resistance to specific pathogens. Look for cucumber and zucchini that are powdery mildew-resistant, tomatoes that are verticillium and fusarium wilt-resistant, and basils that are downy mildew-resistant. Some of these varieties may be open-pollinated, meaning you can save seeds from them year after year. Other varieties may be F1 hybrids, meaning that the seeds saved from them may not be viable or have the same characteristics as the parent plant (this does not mean they are GMO).

My plants are a complete loss, do I have time to replant before the season is over? Yes! You can seed summer squash and cucumbers directly in garden soil now (look for seeds with about 50 days to maturity, listed on the seed packet). Put a pinch of granular fertilizer about an inch from each seed. Try not to plant in the exact same spot as your previously infected plants, as there may still be spores on the soil. Use the same sprays listed above as a preventative measure as soon as you see the plants sprout, and be sure to use a mulch such as straw or leaves, which will also help to keep soil from splashing up on the leaves when it rains.