Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Last week I commented about not seeing much yellow shoulders or uneven fruit ripening problems caused by low levels of potassium in tomato. The lack of problems was due mostly to higher than normal potassium levels in the tomato crop. I heard from several growers who told me that their tomato harvests were not pretty and they sent pictures along to prove that. And I agree that some of our tomatoes look pretty ugly and this is what I’d like to talk about this week.
Some of the ugliness is due to high levels of gold fleck (Figure 1). Gold fleck is caused by calcium crystals being deposited in the epidermal layers of the fruit when certain varieties are under stress. Causes of this stress include high densities of thrips or moderately high numbers of two-spotted spider mites or most commonly when there are consistently high (>90°F day, >68°F night) air temperatures along with high dew points (>68°F). We all experienced these high temperature and humidity conditions, but some fields also had high levels of mites or thrips, making matters even worse.
In some fields I found a great deal of rain-check (Figure 2). Rain check occurs in green and partially ripe fruit when there is rapid fruit growth and the skin can’t expand fast enough. This often occurs when there has been a dry period with high humidity followed by heavy rains. Fruit that has poor foliage cover tends to have more problems with the disorder. These conditions cause small, or, at times, large cracks up around the stem that can expand over time (Figure 2).
Figure 1. Severe gold fleck in tomato
Figure 2. Rain check on tomato
There are ways to reduce the physiological disorders just discussed as well as yellow shoulders and other fruit ripening problems. Selecting some varieties that do well in heat is one, but these often have other undesirable attributes that growers and their customers do not want. Another is using white plastic mulch (Figure 3) rather than black mulch, which will REDUCE the amount of these disorders but they will still occur and the mulch must be put down early in the season. But one way to reduce many of these physiological disorders is by using shade cloths or canopies (Figure 4). These shade cloths can be put up after the first cluster or two of fruit have set if weather conditions indicate prolonged periods of hot humid weather.
Figure 3. White plastic mulch
Figure 4. Shade cloth over a section of tomato row
I have been experimenting with using shade cloth in tomato over the last 5 years and they have worked remarkably well in increasing the marketable yields of many different cultivars of tomatoes by 20-50%. I use a 30% filtering shade (using any more than 30% tends to reduce yields and size of tomato fruit). The shade cloth is draped over the top of the tomato stakes and held down at both ends (Figure 4). I know this does not seem practical, but only the top ¼ of the plant needs to be covered (not shown) which means a grower could use shade cloth with a 4 ft width and as long as they wanted it to be. The shades can be used over and over for many years; the ones I am using have been in use now for 5 years. The shade cloth helps tomato plants come through very stressful weather conditions, i.e., high temperatures with high dew points and even heavy rains in much better shape than plants that were not covered.
Figure 5 shows part of a row (with the red line) that had been covered with shade cloth for six weeks compared with the row next to it which had not – same cultivar planted on the same day. I arbitrarily selected that one section of row for the shade cloth in June. You can see how much better those plants that were covered look than the ones that were not covered. The benefit of using the shades is an increase in quality and size of tomato fruit, rarely in the number of fruit.
Figure 5. Part of tomato row (with red line) that was covered with shade cloth vs others that were not
Figure 6 shows harvest bins of tomato fruit with the bin on the left from plants that were covered from the end of June through July while the bin on the right was from plants (same cultivar) not covered. These experiments were replicated 4, 6, and even 8 times in the field over several years and the results were always the same – an increase in marketable yield each year. Some years it was an 18.9% increase and some years it was a 47.7% increase. Once plants are covered, the shade cloth can stay on the rest of the season until harvest. We sprayed through the shade cloth with fungicides and insecticides. Foliar diseases were reduced for plants under shade compared with plants outside shade. I am not suggesting a grower would shade an entire field, but you might select a few of your cultivars that bring a very good price, but are prone to producing ugly tomatoes during stressful weather conditions and shade those.
Figure 6. Harvest bins of tomato fruit; bin on left from plants covered with shade and bin on right from plants that were not covered.