Last week, we received calls from several watermelon growers who reported that their newly planted watermelon seedlings had died (Figure 1). After closely inspecting the affected plants, we did not find any pathogens. This reminded us of what happened to our cucumbers back in early April in our high tunnel. We will review the cucumber story first, and rethink what might have happened to the watermelons.
Figure 1. Watermelon seedlings planted in a week.
Cucumbers were planted on March 30 in a high tunnel located at the Southwest Purdue Agricultural Center (SWPAC) in Vincennes, IN. The lowest air temperature recorded after planting was 37.5°F inside the high tunnel, which should not be low enough to cause frost damage. However, we lost 40% to 80% of the newly planted cucumbers, depending on varieties. The symptoms were similar to water deficiency-caused plant wilt (Figure 2). Most of the dead plants had intact stems. However, we did find wireworms in the stems of a few dead plants. We drenched the soil with Admire® on April 8. Plants were reset on April 9.
Unfortunately, almost all the reset plants died again. The soil was moist, no pathogens were found, and most of the stems were free from wireworm damage. Why did the plants wilt? Because when soil temperature is below 63°F, the ability of cucumber roots to absorb water is greatly inhibited. In fact, between March 30 and April 10, more than half the time, the soil temperature inside the high tunnel was below 60°F, ranging from 52°F to 67°F. Interestingly, all grafted cucumbers with cold-tolerant rootstocks survived the first planting in the same high tunnel. The dead plants were replanted again on April 14, when soil temperature inside the high tunnel finally stabilized above 70°F, and all the seedlings survived.
Figure 2. Wilt cucumber plants in a high tunnel
Did low soil temperature also cause the watermelon transplant symptoms we observed? Let’s take a look at the bare-ground soil temperature recorded at SWPAC in the past weeks (Figure 3 ). Starting from May 1, average bare soil temperature did not rise above 70°F until May 24. From May 15 to 19, soil temperatures were the lowest. The average bare soil temperature ranged from 60 to 63°F. Coincidentally, the most severely damaged watermelon fields were planted during this time period. Black plastic mulch is indeed expected to increase soil temperature. However, considering the constant rains that occurred in early May, it is likely the plastic was not laid until recently, and the cloudy days did not help at all. What other factors might contribute to the transplant failure? We noticed the transplants did not have a solid root ball when they were planted (Figure 4). These plants might survive if environmental conditions are ideal. However, under adverse soil temperatures, the weak transplants are more likely to suffer first.
Figure 3. Soil temperature at SWPAC from May 1 to May 29, 2016 (data source: iclimate.org)
Figure 4. The watermelon transplant on the left has a loose root ball compared to the transplant on the right. Note that the above-ground plants are of similar sizes.
We cannot control the weather, but there are a few key points we should take away from this year’s lesson
1. If possible, do not plant if the anticipated soil temperature is below 65°F at the time of planting.
2. If possible, use transplants that have developed a solid root ball.
3. Lay plastic early in the season to ensure adequate heat is accumulated under plastic.
What happened in this season also raises a few research questions that warrant exploration
1. Whether watermelon varieties differ in their tolerance to low soil temperature, considering that varietal differences do exist among cucumber varieties.
2. What are the threshold soil temperatures for watermelon transplant establishment?
3. Whether grafting can be an approach to combat low soil temperature in watermelon production?
Hopefully, we can find the answer to the questions in our future research.