We saw two interesting problems on a watermelon field this summer. While both are rather odd, and not likely to occur regularly, they both offer an opportunity to learn from their situation.
The first problem became apparent in July. In the low areas plants began to wilt and die. Fortunately, the grower pulled a sample and sent to Kansas State's diagnostic clinic. They found extensive insect damage which their Entomologist then diagnosed as fungus gnats. They thought this seemed weird but then ended up seeing it a couple more times in the summer and on cucurbits. The lab diagnostician noted, fungus gnats like it wet, and it's been wet.
Also interesting was that this is a greenhouse pest that was carried into the field. In cold and rainy springs, many greenhouses back up with plant material, and this becomes outstanding habitat for fungus gnats. So it is likely that this greenhouse pest was carried to the field and then the mild and rainy weather in June increased their level to the point they were causing recognizable damage. So why didn't the problem become apparent more quickly? This was on the first planting. Answer â€“ probably because his first insecticide treatment was imadicloprid, which is labeled for control of the larvae. But eventually, with the June rain and cool conditions they made a comeback. He then used an insecticide and it checked the spread.
The second problem occurred nearby but was separate. It was a later planting, looking good and about ready for the 1st picking. He then noticed some plants seemed off; smmome wilting, some leaves that should be holding up, kind of flipped over. This coincided to about a week following 3 weeks of that cool and wet weather that stretched from the end of July to mid-August. While they did get a 1st picking, none after that. There was no fruit rot, the symptoms were all with the plant. The plants began to wilt more and more. A sample to the Kansas lab came back showing nothing. On subsequent visits it was the same, the roots/crown looked good but eventually all plants were devastated. Was it a physiological? Or some strange disease? Or something else?
There is a disease syndrome described as "Mature Watermelon Vine Decline". Syndrome was used to first describe this disease, as a pathogen had not been identified. It is typical when the root system is too small or compromised for vines with lots of fruit maturing. We had all the right conditions line up for it to occur. Wet, heavy soil conditions prior to full fruit load. We also had lots of moisture during plant establishment, so that the root system would be shallow. One nearby grower had very similar early symptoms, but the planting hung in there and finished ok. But by then, we were back into ideal summer watermelon weather.
While weather brings on the condition, they have been able to induce it with soil from a field where it occurs. In 2011 a fungus was identified. When it has occurred rotating out of cucurbits for 3 years is advised. Also, varieties resistant to Fusarium wilt may be less likely to have this problem. Lastly, cultural techniques to promote healthy root growth will reduce the incidence. It is associated first in low areas of a field. Purdue University has a fact sheet on this disorder. Let your nearby specialist know if you'd like a copy.
REVISED: November 24, 2015