In the Garden February 2016

By Rhiannon Allen

As spring peeks around the corner, gardeners’ thoughts turn to summer. Soon we will decide what to plant, head to the seed racks to purchase new seeds and prepare our garden beds.

At this time, it can be productive to consider ensuring plant health. The end of February is a good time to start digging in compost, manure, slow-release fertilizer, mycorrhizal fungi and other soil amendments so that spring rains can work their magic in time for planting. But I’m also thinking of further ahead, to late summer and early autumn vegetable harvesting, and what I can do now to ensure a healthy crop.

An issue that West Coast gardeners confront is late blight, a problem which emerges as temperatures drop and humidity rises in late summer and early autumn.

This is a disease that affects members of the potato family. Its scientific name is Phytophthora infesta– what a mouthful. It’s a fungus-like organism that can destroy entire potato or tomato crops.

In late August, if you look closely at an infected plant, you can spot a darkening or blackening of the petiole, the stalk that attaches a leaf to the stem. Soon, irregular, usually grayish blotches appear on leaves. If you look at an affected leaf closely, you might see visible mold on the underside.

With tomatoes, this infection opens the way for secondary bacterial infections that can rot the tomato fruit. With potatoes, the original blight can directly rot the tubers that are encased in the moist ground, leaving each potato a slimy mass.

Late blight is the organism responsible for the 1880s Irish potato famine. Fortunately, with modern supermarket and agricultural systems, we are not affected in the same way as the Irish population was.

However, even if we are not dependent on our home gardens, what supermarket tomato or potato can delight us as much as homegrown produce? One of the pleasures of summer is a tasty potato or luscious sun-warmed tomato.

So what can be done, starting now, to avoid losing a crop to late blight? The first step is buying plants or seeds that are blight-resistant. For potatoes, unless you are careful in preparing your own seed potatoes, make sure that you buy certified seed potatoes harvested from disease-free plants.

If you insist on using grocery store potatoes to start a crop, use only very healthy-looking organic potatoes. Organic farms are not immune to late blight, but at least you’ll get a potato that does not contain substances such as sprout-inhibiting chemicals.

If you want to be extra cautious, see if you can purchase seed potatoes of varieties that have been developed as blight-resistant.

For tomatoes, would early-fruiting varieties fit your bill? Since late blight does not generally appear before the end of August, tomatoes that produce early in the season generally ripen before late blight becomes a problem. Many cherry tomato varieties fruit early, although the first time I had a late blight problem, it appeared on an early-ripening gold cherry tomato plant.

If you want larger resistant varieties, Veseys sells seeds for other blight-resistant tomatoes. “Resistant” doesn’t mean “immune,” but it’s worth a try.

Once you get seeds, what can you do to ensure a blight-free crop? The first key, if you can manage it, is to plant in new, high-quality soil. Commercially purchased soils are generally free of Phytophera spores, which can persist dormant in the soil for years. This works well if you plant in containers.

Container planting even works for potatoes, which can be grown in containers, bags of soil, or flexible containers made of recycled plastic. If you must plant in old, possibly infected soil, consider mulching heavily so that water will not splash dormant spores up onto leaves.

For tomatoes, I like to use red plastic sheets, available via mail order or in some nurseries, since the red color has been shown to increase yields of tomato plants.

Another key is providing a low-humidity environment, since Phytophthora flourishes in damp conditions. Space plants well. Plant under south or west-facing eaves if you can. For tomatoes, provide supports that allow for air circulation. Water the base of the plant, so that leaves are never damp. The last thing you want is dreaded dormant spores landing on a moist leaf, where they will awaken.

If you are not able to prevent all possible routes for Phytophthora, then you can use a copper spray in late summer before the disease appears. Copper is toxic to many microorganisms like Phytophthora. It can also be toxic to macro-organisms in high doses, so do not use it during harvest time.

Finally, do not compost infected plants. Infected plants will be rife with spores just ready to sleep the winter away and attack next year’s plants. Put infected plants directly into the garbage.