Maturing Potatoes in Western Washington

Why is it so hard to get skin set on a potato in Western Washington? This is a question that has confounded growers for years. So many times they just shrug it off by saying it’s the weather and you can’t change the weather. This is partially correct but let me explain the whole process and how you can change it.

Like all plants, potatoes are driven by five hormones. These are Indole-3-acetic acid (IAA), cytokinin (Cyt), gibberellic acid (GA), ethylene (ET), and absisic acid (ABA). The first three, IAA, Cyt, and GA are considered to be growth hormones while ET and ABA are considered to be stress and maturation hormones. The growth hormones are dominant at the stage of growth from seed germination, or sprout development, to seedling stage up to vegetative stage. All three of these hormones will be dominant at one time or another through bulking up to maturation. Prior to maturation, a hormone shift from IAA, Cyt, and GA over to ET and ABA must take place in order for the potato to be mature and the skins set at harvest.

That’s what needs to happen, but now let’s look at what does happen. In Western Washington, with our marine climate and high organic-matter soils, our potato plants are still dominated by GA when ABA dominance is needed. Nitrogen released from organic matter promotes the GA dominance so it must be blocked. This is where high rates of molybdenum come into play.

Molybdenum (Mo) works by blocking the impact of nitrate release and reducing GA dominance. When this occurs, ABA is the dominant hormone and maturation begins. Without this process, you will wait 40 to 50 days for skins to set leaving the tubers exposed to silver scurf infection. Dr. Phil Hamm from Oregon State University has said many times that to prevent silver scurf, growers need to get the tubers out of the ground as soon as possible.

To aid in the maturation process, two different directions can be taken. One direction is to apply a small amount of Stoller’s CoMo Classic (2% Cobalt, 3% Molybdenum) throughout the growing season. This will keep the plants from becoming overly dominant by GA later in the season. The second method is to apply Stoller’s Phos Moly (3-28-0-4%Mo) at a rate of one quart per acre three weeks before vine kill. At this rate of Mo, the hormone will force the ABA to increase which will reduce GA.

We cannot change the weather, climate, or organic matter but we can change how they impact the potato crop. No magic, just good science.