It is not unusual for watermelon wilt to occur in the late season in our region. The plants are loaded with fruit and are sensitive to many stresses. In some cases, it is clear pathogens cause the problem, such as late-season fusarium wilt. While in other cases, it is hard to associate the symptom with a particular pathogen. In these cases, we generally call the symptom late-season watermelon vine decline. The fact that the symptoms often appear after heavy rainfall and first appear in low, poorly drained areas indicates waterlogging stress is involved.
When waterlogging occurs, it displaces oxygen from soil pores and promotes oxygen depletion by roots and microorganisms. The anaerobic condition increases the concentration of toxic chemicals in the soil, which results in root injuries and slow growth. The damage reduces the roots’ capability to take up water and nutrients. Above the ground, plants close stomata to avoid water loss, leading to a subsequent reduction in photosynthesis. The typical plant symptoms include wilting, yellowing leaves, and leaf senescence.
Waterlogging stress likely contribute to plant wilting symptoms observed in watermelon fields after heavy rains. But these symptoms do not show in all fields that receive the same amount of rain. More importantly, the symptom does not always associate with soil type. We have seen the symptoms in areas with well-drained soil.
Under the same natural conditions, plants vary in their tolerance to waterlogging stress. Certain cultural management practices may also make plants more susceptible to the stress. It is reasonable to believe that plants with more intensive, deep, and widely spread root systems exploring larger soil profiles are more likely to tolerate stress conditions better.
Cultural Practices Known to Affect Root Growth
A single cultural practice is not likely to mess up root development. Still, it is possible that multiple practices combined create a condition that limits the plants’ capability to develop a robust root system. Transplanting instead of direct seeding is known to affect tap root growth. Using black plastic mulch combined with constant water application through drip tapes is sometimes found had shorter watermelon tap root compared to the ones grown on bare-ground. Although we realize the limitation of using transplants and plastic mulch, they are used in watermelon production because of their apparent benefits. In most cases, we saw benefits of using these practices rather than negative effects. The watermelon industry is not likely to abandon these practices. Thus exploring other cultural practices that may affect root growth is becoming increasingly needed.
Watermelon Roots Distribution and Comparing with Other Crops
Cucurbits generally have strong, rather shallow taproots and numerous strong, horizontally spreading laterals in shallow soil. Documented in Root Development of Vegetable Crops (Weaver and Bruner, 1927): when the watermelon plants are just beginning to vine, the tap roots may be at a depth of 12 inches, but horizontal branches may reach 2 to 3 feet in length, and secondary branches are abundant in shallow soil. When vines grow 4 to 8 feet long, the tap root only grows slightly deeper, while the lateral roots may spread to 5 feet. When maturing plants develop vines 15 to 18 feet long, a few laterals could extend outward 18 to 21 feet from the base of the plant.
To compare with watermelon roots, tomato and strawberry root drawings are included in the above reference. A noticeable difference in tomato roots is that laterals may extend outward up to 2 feet, but instead of spreading, they turn downward and reach depths up to 3.5 feet. Strawberry roots also do not spread. The dense network of fibrous roots is concentrated in the top 10 to 12 inches of soil under the plants.
Although current cultivars and production systems may change the extent of root expansion in the above crops, the general trends still apply.
Cultural Practices that May Affect Watermelon Root Expansion
As the root patterns vary greatly among different crops, some popular practices could apply differently to the other crops—for example, bed height. Plasticulture strawberries are well known to be best grown when beds are at least 8 inches tall. The tall beds prevented waterlogging from happening in most of the root zones. Tomatoes and peppers are also better grown in relatively tall beds. It avoids waterlogging and soil splashing to the lower leaf canopies. In contrast, a taller bed may not be the best option for cucurbit crops. As the primary roots tend to spread out, it is possible that tall, raised beds could negatively affect root expansion beyond the bed area. Row middle cover cropping might be another example. The system greatly prevents soil erosion and provides windbreaks to protect young seedlings, as well as many other benefits. However, the green living mulch in row middles might compete with watermelon for moistures stored in soil profiles outside of the bed and discourage roots from expanding to row middles. Heavy irrigation under plastic mulch will also likely deter root expansion to the row middle to explore additional water sources.
Here we discussed a few cultural practices that may influence watermelon root expansion and plants’ tolerance to waterlogging stresses. In a typical year, these improved cultural practices may benefit watermelon growth and yield, but under extreme weather conditions, especially excessive rainfalls, the most protected and intensively managed plants tend to be the least resilient to extreme conditions.
We understand some of the cultural practices have been adapted in other watermelon growing regions because of the various benefits. But how to adapt them to the Midwest conditions, the climate, and the soil? How to fit them in our overall production system? Thoroughly evaluations of each of the new practice’s pros and cons, and their combined effects in our watermelon production system is critically needed.