It’s that time of year when many gardeners are eagerly awaiting their first ripe tomato fresh off the vine. Unfortunately, just as tomatoes begin to ripen they often fall victim to the notorious late blight fungus. Late blight is one of the most devastating diseases of potatoes and tomatoes worldwide. It was responsible for the Irish potato famine of the 1840s and continues to be a challenge to commercial potatoes as well as home gardeners. With tomatoes, in less than four days, this aggressive disease can result in the complete destruction of not only green and ripening tomatoes but the entire tomato plant as well.
Our warm summer temperatures combined with dew, fog and rain are ideal conditions for the development of this disease. Late Blight appears on potato or tomato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish black. During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears. Infected areas on stems appear brown to black and entire vines may be killed in a short time when moist weather persists.
On green tomato fruit, Late Blight fungus produces large, firm, brown, leathery-appearing lesions which are often concentrated on the sides or upper fruit surfaces. If conditions remain moist, abundant white mold will develop on the lesions and secondary soft-rot. Bacteria may follow, resulting in a slimy, wet rot of the entire fruit.
Fortunately, home gardeners can do a couple of things to prevent their tomato crop being infected. First, placing a temporary roof over the plants thereby keeping moisture off the foliage will do much to prevent infection. Avoid wetting foliage when irrigating, especially in late afternoon and evening. Staking and pruning plants to provide good air circulation will also help.
Fungicide sprays, applied before disease begins, is the only method that can prevent complete crop destruction. Fungicides registered for use include Bravo Weather Stik and Kop-R-Spray. Fungicides applied now, according to labeled directions, will protect plants from infection.
The disease is equally devastating to both tomatoes and potatoes. The fungicides Bravo, Maneb and Mancozeb are all registered for use on potatoes and provide an effective protectant against the disease when used according to labeled directions.
It’s almost September and our tomatoes are simply not ripening! What’s wrong? Is there anything we can do to speed up the ripening process?
If your garden tomatoes look like they’ve been placed on hold, you’re not alone! Tomato fruits go through several stages of development during their maturation process. During early stages, the fruit continues to grow in size and remains green, typically requiring 40-50 days. Once the fruit has reached full size (called “mature green”), changes in pigment begin to take place, causing the green to fade to light green then to the appropriate pigments for that particular cultivar, be it red, pink, yellow or orange.
Ripening and color development in tomatoes is governed primarily by two factors: temperature and the presence of a naturally occurring hormone called “ethylene.”
The optimum temperature range for ripening mature green tomatoes is 69-77°F. The further temperatures stray from the optimum, the slower the ripening process will be. Furthermore, when temperatures are outside the optimum range for extended periods, conditions may become so stressful that the ripening process virtually halts.
At the same time, tomatoes do not produce lycopene and carotene, the pigments responsible for ripe tomato color, when temperatures are above 85 °F. Surprisingly then, extended periods of extreme heat cause tomatoes to stop ripening. The fruit often appear yellowish green to yellowish orange.
There’s not much you can do but wait out the weather. As temperatures become more favorable, the ripening process should get back on track. We should have quite a few more weeks of good growing weather before a killing frost arrives.
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Overloaded with Zucchinis? Gardeners with extra produce can donate it to the Montesano Food Bank. Call (360) 249-4163 for information on drop off time and location.
This column is printed as a service of the Washington State University Extension Office at Elma. Donald D. Tapio is the regional gardening and farming specialist for Grays Harbor and Pacific counties. He can be reached at (360) 482-2934 or firstname.lastname@example.org.