Seed Potatoes and Late Blight

Jesse Fulbright, Liberty County Extension

Earlier this month orders went in through the M.S.U. Potato Lab about bulk orders of seed potatoes for our communities.  Sometimes this is handled through the Extension office, other times through your local garden center.  Either way, the time is approaching when people will be planting potatoes. 

Montana is one of the top five seed-potato producing states.  The state’s seed potatoes are prized because growing areas are somewhat isolated from airborne spores of diseases such as late blight.  To protect the industry, Montana only allows potatoes that originate in Montana to be grown as certified seed, and requires all seed potatoes to be inspected at a shipping point.  As seed potatoes are an important crop in Montana, it takes everyone, including the home gardener to help keep diseases, such as late blight in potato, in check.

Late blight infection is characterized by brown to purplish slightly depressed regions on the skins.  Upon peeling back the skins, reddish-brown to dark brown granular tissue is apparent.  On very young leaves, irregular water-soaked lesions appear.  Lesions are dark brown to black and can appear small at first.  A light green halo often appears around the lesion.

Even though this year’s home gardener hasn’t planted their potatoes yet, it is important that people go into the season with their eyes open to potential diseases such as late blight.  No potato varieties have resistance to all late blight strains, but some have resistance individual strains.  Good soil drainage and proper plant spacing for optimal air movement are desirable.  Destroy suspect potatoes, but not by composting.  Deep hilling, or bringing loose soil up around the vines and lower leaves where the potatoes will form can be used to protect tubers from blight spores.  Make sure watering occurs at a time of day so the leaves dry during the day.  Avoid excessive fertilization.  Fungicides are also an important part.  Use a labeled dose at recommended intervals. 

For all the details about late blight, contact your local county Extension office for a copy of the MontGuide titled, “Late Blight in Garden Potatoes.”