Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
These last few weeks have been particularly difficult on our crops with the consistently high temperatures and humidities (dew points really) along with being in a drought, all of which can put great stress on plants to produce high quality fruit. This is when using a 30% black shade cloth can greatly increase the marketable fruit that the plant produces (Fig. 1). I have talked about using shade cloth before and how well it has worked over a 5-year study period to increase marketable fruit harvest of tomatoes and peppers. The University of Delaware has found similar good results when they have examined shade for peppers, lettuce and tomatoes. Shade on tomatoes probably would have been best started in the third week of June this year, although it usually goes on the first week of July. If the rest of July is average it would pay to put on a 6 ft wide however long you wanted shade cloth over your most prized tomato variety. You will see the difference between being covered and not covered after 4-6 weeks (Fig. 2).
Figure 1. Black 30% shade cloth treatment on black plastic in tomatoes
Figure 2. Tomatoes with and without shade cloth applied for 6 weeks (1 July – 15 August)
Other mitigating methods I and other researchers have looked at include using white plastic mulch instead of black and increasing potassium levels in tomatoes. Problem is, you have to plan on using the white plastic mulch way ahead of time and while it does help as does adding greater levels of potassium, these two practices do not do as much good as using shade.
One other thing I’ll mention that might help mitigate our high temps and humidities is by using biostimulants. I have had mixed results with using various biostimulant products that are for sale but have seen some positive response from using bacteria such as Bacillus spp., Pseudomonas spp. and Trichoderma harzianum, as well as certain fungi such as Beauveria bassiana. In figure 3 you can see one rep of a trial with a biostimulant treatment on the right-hand side of the picture. You can see the extra growth of tomato foliage compared to the middle treatment which was covered by shade cloth or the row on the far left of the picture which was the control—no shade or biostimulant used just a fungicide. I often would see a plant ‘stimulated’ with extra growth when stress factors, such as high temperatures or water stress affected the plant. Unfortunately, this stimulating effect is not consistent whenever the biostimulant is used. Other researchers in the US and elsewhere have found similar results. The middle row that had the shade has very nice green color and out yielded the biostimulant treatment (by 18%) as well as the control (by 37%) in this study. But I still think it would be worth growers experimenting with biostimulants on their farm to help their vegetables overcome such stressful climatic conditions like we are having now and in the future.
Figure 3. One rep of a trial where a biostimulant (right row) was compared with using shade cloth (middle row) or a standard treatment of no shade or biostimulant-control (left row). All treatments were sprayed with fungicides throughout the trial.