Jerry Brust, IPM Vegetable Specialist, University of Maryland; email@example.com
Figure 1 shows part of a tomato field that I wrote about a few weeks ago concerning how environmental stress (high temperatures and drought) on the plants greatly affected plant viability and fruit quality. Figure 2 is the same field today (Sept 22) with new green growth, lots of flowers and fruit, some of which will make it to harvest. This field has not been irrigated for the past 3 weeks and it is still growing well. It is amazing how vigorous the plants are without the stress of the environment and a heavy fruit load. Too often I think growers do not give environmental stress on their plants enough “credit” for some of the problems they see later in the season regarding fruit set and fruit quality.
One of the difficulties I have been having about some fruit ripening problems we see each year in tomatoes is why the problem seems to occur at about the same time of the season regardless of the maturity of the tomato plants (although plants with a heavy fruit load tended to have more problems than those with lighter fruit loads). One thing I was looking at was potassium (K) levels in plant tissue, which were greatly reduced in plants that were having the fruit problems. But why was the K and sometimes phosphorous (P) levels dropping in these plants at about the same time? One possibility I found was that when root zone temperatures (RZT) reached 82o F or greater the plants slowed their uptake of many nutrients including K, P and ,at times, calcium according to leaf tissue analysis. This phenomenon usually occurred earlier in the season and more severely in high tunnel tomato production systems. The problem is that sometimes as the levels of K decreased it was not always correlated with an increase in fruit problems. There appears to be additional factors involved besides lack of some plant nutrients. Would cooling roots somehow help off-set the heating of the root zone and could irrigation water from a well help this? From preliminary studies that I am still working on it appears the answer is no to both. Plots that were irrigated with well water vs. those irrigated from pond water seemed to have a slightly greater reduction in nutrient uptake, even though the RZT drop was greater temporarily.
One other study I worked on this year showed that I was able to reproduce “thrips” or “mite” feeding injury on tomato fruit with no (actually very low) thrips or mite populations being present (Fig 3). This was done by stressing plants that had a heavy fruit load. The more stressed the plants were (including RZT) the more the “thrips damage” showed up. Plants that were not stressed had little or no “thrips damage”; all plants had the same density of thrips and mites on them—very low. What then is causing this damage to tomato fruit? I still do not know. When I talk to hydroponic tomato growers they recognize this damage as nutrient imbalances and not as insect. This would make sense as the plants in the field become stressed the malady suddenly appears. I’ll talk more about this in winter meetings.
Figure 1. Stressed tomato plants, August
Figure 2. Same plants recovered, September
Figure 3. Damage to tomato fruit usually attributed to thrips or mites, but with no thrips or mites present