Dr. Eric T. Stafne, Extension and Research Professor, Mississippi State University
I like to watch movies. In fact, I watch part of one every morning while riding my stationary exercise bike. The other day I was watching one and a character said, “What you don’t know about me could fit inside the Grand Canyon” (this is the G-rated version of the quote). Most days I feel that way about life in general, but some days it’s more relevant to a specific research project. In this case, white drupelet disorder (WDD) in blackberries.
There are some things we know, or think we know, about the disorder – it is manifested from a genotype by environment interaction, high temperatures seem to exacerbate it, and light intensities are also likely involved. How do we know these things? I did a study in Poplarville, Mississippi on three different cultivars, Chickasaw, Kiowa, and Sweetie Pie, in an open field and under shade (https://journals.ashs.org/horttech/view/journals/horttech/27/6/article-p840.xml). We found that the amount of WDD differed among the cultivars, hence confirming genotype x environment interaction. We also saw that when temperatures increased the amount of WDD increased as well, thus temperature is involved. And, when we grew the same cultivars under shade almost all the WDD vanished, ergo light intensity is also likely involved.
So, that sounds like we know how to avoid it by choosing the right cultivars and reductions in heat and light intensity. Using a trellis system such as the rotating cross arm trellis has been shown to reduce WDD. Fernandez et al. did work that broadly shows that WDD may be as a result of plant stress https://smallfruits.org/files/2019/07/2018R-13.pdf. And still, on the genetic side of things, the question that continues to haunt me is: which are the right cultivars and what drives the susceptibility to WDD? These are not so easily answered, at least not yet. Dr. John Clark once told me that Dr. Moore said WDD could be avoided in the selection process, but Clarksville, Arkansas may not be the best environment for WDD to show up on a consistent basis.
In my own work I would agree with the assertions of Dr. Fernandez. Recently, I finished up a three-year study analyzing the effects of additional nitrogen fertilizer on the response of WDD in Sweetie Pie (a susceptible cultivar). Since we just concluded harvest last week, I have not completely analyzed the data. However, from the first two years of data it appeared that additional nitrogen application (in this case, urea) suppressed WDD, but only during times of stress (e.g. high temperatures). I will know more as I wade through the third year of data.
My goal is to understand the disorder better. Why do only certain drupelets turn white? Why do some drupelets appear only half white, half red? Why do they seem to be randomly placed on the fruit? Could other factors (i.e xenia (pollen effect)) be involved? Whatever the answers are to these questions, I believe we should be cautious in saying what causes WDD until more studies are conducted. Certainly, answers will be gained from my most recent study, but probably not enough to fill the Grand Canyon.