Mildew Resistant Cucumbers: Possible, or Just a Fantasy?

Every summer, around mid-July or early August, I start hearing complaints about cucumber, squash, and melon plants whose leaves have acquired that gray-ish look that goes with Downy Mildew.  Productivity slows, and the plants die. Out of desperation (possibly) and a longing for more fresh cucumbers and squashes and melons (likely), many gardeners treat their ailing plants with an array of sprays, from baking soda mixtures to compost teas to official fungicides purchased in the pesticide section of a garden center. All of these can slow down the spread of Downy Mildew to some extent, but none will actually cure the plants.

In general, the inability to cure the problem is a good reason to grow the most disease-resistant varieties we can find.  UGA Extension publications list varieties that are good to grow in our area, and a big reason varieties make it onto the list is that they show pretty good resistance to diseases that are common here in the Southeast.

These are the currently recommended cucumbers: Salad Bush Hybrid, Bush Crop, Fanfare, Burpless Hybrid, Diva, Marketmore, Straight Eight, Sweet Slice, and Sweet Success.


When I first started gardening here, about 25 years ago, I tried several different varieties of cucumbers before settling on Straight Eight (notice that it's on the recommended varieties list) as the best for my yard, but I later switched to Straight Nine, a selection of Straight Eight from Sand Hill Preservation, and each year I find that it out-performs most other varieties in local gardens. Part of its charm is that it is slower to succumb to both Downy and Powdery Mildews.

This year, though, I am going to grow another cucumber variety alongside Straight Nine. I found the seeds through my trip to Monticello with a couple of gardening friends last September. We had a great time meeting other attendees at the Harvest Festival and asking about their gardens, but we also met some seed producers, one of whom was a young man from Commonwealth Seed Growers. After a brief conversation with the grower, we bought a few seed packets and added our names to his mailing list, and then we moved on.

When the catalog showed up in the mail, though, I saw that a big focus of the group is identifying and producing varieties of cucumber/squash/melon family plants that resist Downy Mildew. Talk about a great gift to gardeners and small farmers here in the Southeast!

My seed order from that group includes a packet of the cucumber variety DMR-264, a selection released from Cornell University. The variety has shown excellent resistance to Downy Mildew on the farm where it is being grown out for seed, and I am very interested to see how it will compare to Straight Nine.

I have friends, including some at local Community Gardens, who have been looking for mildew-resistant squashes, melons, and cucumbers, and it seems possible that Commonwealth Seed Growers could have the seeds of our dreams.

I surely am hoping so. Meanwhile, next weekend, I will be looking into more new varieties and other gardening discoveries at the Georgia Organics conference. It will be great to spend some time with other people who are so very focused on growing good food.

Squash Downy Mildew

It has been hilariously rainy here, and the garden is having some of the expected disease problems that go along with all the wet soil and humid conditions. For example, a volunteer butternut squash plant growing among the tomatoes is showing signs of downy mildew on its oldest leaves.

The front of the leaf looks unwell, but not spectacularly bad: it is yellowing and is showing some small brown spots inside bright yellow halos. This alone would not be enough to really nail down a diagnosis. (Sorry for the quality of the photos - I was having trouble with the lighting.)

The backside of the leaf is the clincher: it shows the development of the purplish spores of the mildew.

A 2009 article about Downy Mildew, by Debbie Roos, out of NCSU's Growing Small Farms program, includes a useful set of suggestions for managing downy mildew in cucurbits (cucumbers, squash, melons) using organic-growing methods:

"Cultural strategies can help prevent downy mildew. Plant resistant varieties. Plant in locations with good air circulation. Use drip irrigation to minimize leaf wetness.

There are a number of OMRI-listed products purported to help control downy mildew in cucurbits: copper, neem, biofungicides (e.g., Serenade®), peroxides (e.g., OxiDate®), and bicarbonates (e.g., Kaligreen®). According to Dr. Louws, research has shown that copper is the best organic option, but only on the crops that show little to no symptoms. He said if the infection is far along not to bother spraying because it wouldn’t do much good. Spray early in the morning to avoid phytotoxicity problems caused by spraying in the heat of the day. If the disease is present on the farm, a prophylactic application of a copper product can be made to curcurbit crops that show mild or no symptoms. If the weather does not favor the disease (which likes it warm and wet and humid), then the copper is more likely to suppress the disease. In other words, the copper may help but it may not be enough. (See Pesticide Use Guidelines)."

Notice the mention of weather that is "warm and wet and humid"? That is exactly what we have going this month, so this one volunteer squash might not be the only plant in Cobb County to be experiencing the problem. It may be helpful (though disheartening) to know that Powdery Mildew isn't the same as Downy Mildew, so that treatments that work for Powdery Mildew, which many local gardens dealt with last summer, might not work on this different mildew/fungus. (The ATTRA article on Downy Mildew in cucurbits, by George Kuepper, offers similar suggestions to those in the above NCSU article, with caveats that don't appear in the Roos' version, in case anyone is interested in looking at an additional source.)

It is a little too late to plant resistant varieties this year, but if anyone has a garden that is particularly plagued by this fungus, selecting a resistant variety for next year might be a good idea. The bad news is that I have been unable to find a good list of resistant cultivars. Apparently, several cucumbers have decent resistance to Downy Mildew, but very few squash do.

However, it is still possible to improve air circulation around existing plants, by pruning or removing nearby other vegetation, even though we can't do much about the rain.

I would whine about how the weather this year is unusually bad for gardening, but it seems to be bad, in one way or another, pretty much every year. There will be a deluge, or a drought, crazy high winds, unusual high temperatures, or unusual cool temperatures, tornadoes, hurricanes or their remnants, hail, falling trees that come across the top of the house and smash parts of the garden.... you get the picture. No year will be The Perfect Gardening Year. Amazingly, we still get plenty of good food out of the garden, and I still love to be working in the yard on my little patches of crops.

Hope that all the other gardens and gardeners out there are growing well!

Tomato Problems Abound

The tomato-ripening has been slow to begin, but it finally is coming along at a faster clip. The Cherokee Purple tomatoes are big and beautiful; however, the plants on which they are growing look terrible. I wasn't able to get a good picture of the tops of the two plants, partly because the plants are so tall and partly because it keeps raining. Aiming up results in a wet camera lens!

Many of the leaves are bedraggled, and there is marked wilting of some individual stems, while other stems continue to look just fine. I sliced into one of the wilted stems and saw brown in the vascular tissue (located between the green skin and the white, central pith). Those indications, along with the fact that the tomatoes continue to look good, leads me to believe that the problem is one of the soil-borne wilts, probably Fusarium. That is a huge relief, because I have heard that Late Blight (a very bad tomato disease) has been "going around."

Great-looking Cherokee Purple tomato, finally ripening, on a very sad plant.  PHOTO/Amy W.

Ironically, the Costoluto Genovese tomatoes also look good, and also on a very raggedy-looking plant, but it has a completely different problem; it has a leaf-spot disease. I'm thinking that it is Early Blight, because the brown areas on the leaves continue to expand (slowly), and they get the typical concentric rings as they enlarge.

Costoluto Genovese tomatoes, on a plant with a leaf-spot-type fungus.  PHOTO/Amy W.

For the Cherokee Purple tomatoes, my current plan is to harvest all the big tomatoes and then pull up the plants. My next patch of bush beans will go in their place. I just need for it to stop raining long enough to manage the task!

The Costoluto Genovese tomatoes aren't quite as far along. I keep hoping for a dry day (notice a theme here?) on which I can go out and trim away all the "bad" leaves and then see if there is anything besides fruit left to salvage after I complete the pruning.

While I was in Texas, the rain gauge out in the garden accumulated more than five inches of rain. The tomato-disease problems aren't a big surprise, considering how wet it has been all spring and summer. However, they are annoying. Since the forecast is for more, and yet more rain, I am thinking that it is time to come up with a Plan B. Wish me luck?

Meanwhile, there is a little lesson here about these two varieties of tomatoes: Costoluto Genovese seems to resist Fusarium wilt, and Cherokee Purple seems to be resistant to Early Blight. In future years, that information might come in handy.

I hope that all the other gardens out there are having fewer tomato problems!

Plant Health Management

I went to a workshop about organic farming/gardening down at Fort Valley State University this past week. Most of the speakers addressed the administrative end of things - how to get a farm certified as being organic, who needs to get certified, who qualifies for financial help and where to get that help. That was all great information, but that wasn't all we heard about.

Dr. Elizabeth Little, a plant pathologist with UGA, was also there, and her talk was very different. She said some things I've been trying to tell people for years, but she said it all better and with the authority of a PhD who has been doing actual, official research into the topic. The gist of it was this:

There are no organic products that REALLY work for disease management; switching to organic farming or gardening isn't about simple substitutions of one chemical for another. Essentially, in organic operations, it's all about prevention.

The organic farmer/gardener takes a systems approach to plant health - based on fertility, plant selection, crop rotation, sanitation, and site selection. The organic system also relies a lot on biological interference with disease; by promoting a good ecological system in the soil (a wide range of fungi, bacteria, and creepy-crawlies), the organic gardener/farmer heads off many potential problems.

Any problems that crop up typically indicate an underlying health issue.

She emphasized that healthy plants resist disease, and that we can promote good root growth and beneficial microflora (and by doing so improve plant health) through providing compost and other organic amendments, by mulching, by reducing the amount of tillage, and by using cover crops.

Encouraging predator insects, parasitoids, and microbes as allies was also brought up. Relying on an ecological approach of planting flowers that are attractive to these beneficial organisms was part of the biological approach of disease prevention. Some diseases are in the wind and can't really be intercepted or diverted, but others are spread through the feeding of insects, kind of like the way mosquitoes spread disease from one animal to another. The "beneficials" help by attacking the disease-spreading insects.

When I spoke with Dr. Little later in the day, when we were touring the campus farm, she emphasized the "right plant in the right place" approach to plant health in a great example: She said that she had seen lone tomato plants out in the full sun, with good mulch around them, well-fertilized and mulched, and they were completely unblemished - no signs of disease anywhere - when other tomato plants in the area were definitely ailing.

It was great that she chose Tomato as her example, because that seems to be the garden vegetable that is most affected by disease, in a way that causes the most distress to the gardener, in Georgia. Usually, when someone asks me about disease in the garden, we end up talking about tomatoes.

I left the workshop feeling extra-motivated to keep emphasizing the importance of all the little steps - using compost, paying attention to plant varieties and their disease resistance, making sure there is adequate sunlight for the plant, and using mulches and cover crops.

All in all, it was a great day.