Rose Ogutu, Horticulture Specialist, Delaware State University rogutu@desu.edu
A visit to high tunnels reveals a common occurrence of yellow shoulders and blossom end rot in high tunnel (HT) grown tomatoes this season. The shoulders of tomatoes refer to the rounded portion of fruit close to the stem scar attachment to the vine. Despite the fruits growing normally, the green pigment on this portion fails to turn to the desired color (Figure 1). Leaving the fruit to hang on the plant for longer periods does not improve the situation (Figure 2). This phenomenon reduces fruit quality. Some tomato varieties are more resistant. More research is needed to identify, succinctly, which varieties are more resistant.
It is evident that nutrition programs geared towards reducing yellow shoulders also lead to a great reduction in blossom end rot.
Additionally, temperatures have been soaring in high tunnels. Temperature data collected from four high tunnels in Kent County (Delaware State University- Smyrna Outreach and Research Center, Townsend and Hartly) and two high tunnels in Sussex County (Seaford and Laurel) registered more days with temperatures over 95°F between early May to mid-June 2015 compared to the same period in 2014.
At the same time last year (2014), high tunnel-grown tomatoes from the same high tunnels were marvelous.
Figure 1. Tomatoes growing normally on vines but ripening is not normal. Photos taken June 22, 2014 in Seaford, DE
Figure 2. An attempt to leave tomatoes on the vine to continue ripening leads to tomatoes rotting on the vines.
What Causes Yellow Shoulders in Tomato?
- High temperatures of over 95°F
- Low exchangeable potassium levels and high alkalinity. Potassium should be above 3% of dry matter when tissue analysis is done at flower initiation.
- Excess magnesium on relation to calcium.
Preventing Yellow Shoulders
- Shade the high tunnel
- Avoid too much alkalinity which leads to potassium and calcium level problems. Soil pH levels should be between 6.0 and 6.8 for tomatoes. Keep irrigation solution pH at 6.2 to 6.5 by using sulfuric acid (conventional growers) or powdered citric acid (organic growers).
- Amend your soils accordingly (Add potassium and calcium based fertilizers eg. Potassium Sulfate, Potassium Nitrate, Potassium Carbonate, Calcium Nitrate, Calcium chelate). Use fertilizers with higher potassium levels. Fertigated and foliar nutrients are necessary. Adding more potassium when the plants are already fruiting does not make a big difference.
- Amend soils with rich composted organic matter.