By Erin Schneider, Hilltop Community Farm
and Dr. Ruth Genger, UW Madison Department of Plant Pathology
Admittedly, I have a bit of winter amnesia when it comes to potatoes. I turn to daydreaming in the fields of season’s past, wondering what worked and how did I come to that decision when assessing my potato plots? If you’re like me, when I first started participating in the organic potato variety trial project, the disease and insect rating criteria was a little intimidating. What do I look for and what do I compare this to? Is that black lined iridescent green bug a friend or a foe perched on the underside of Red Maria? I’m still learning and pictures have helped.
I’ve put together a photo journey for evaluating potato varieties below. As you walk the virtual fields, what do you notice? Are there pieces missing? Resources that have been helpful for you? Pictures that you have, or that we need that would be helpful in understanding what to look for, how to rate? How do the ratings form rate for you?
Step 1: Plan Ahead and Prepare
Tools of the trade: Just you, your clipboard with datasheets and a pencil are all you need for ratings. Some growers also take their camera/smart phone and take photos, get video footage to document.
Rating varieties should take 30 minutes to an hour, depending on how many varieties you are growing. Once I plant my potatoes, I map the variety trial plantings and I make a note in my calendar 6 weeks and 10 weeks into the season for when I need to rate potatoes.
Step 2: What do you mean by plant vigor?
Comparing and rating plant growth in relation to each other helps me understand plant vigor. For evaluating varieties 1 would equate to very feeble plants (in relation to other varieties planted) and 5 would mean the plant variety stands out for its fast, vigorous growth.
Step 3: Row Closure – taking the birds eye view of the canopy. Ratings relate to the percentage of plot for which the canopy is touching between rows.
Step 4: Weed Tolerance
Different potato varieties may exhibit different weed tolerance thresholds. It’s important to note, that growers should utilize their usual weed management strategies, whether it’s mulching/hand weeding, or cultivation/mechanical, combination of the two. The idea is to assess varieties that work for your farm/markets and see if there are specific nuances in organic systems. Ratings range from 1 where dense weeds are shading most of the potato plants to 5, the best case scenario (only a few small weeds under the potato canopy – an organic farmer’s dream).
Trickier questions! and a simpler approach to them
Now we come to some phenomena that are more difficult to assess – insect and disease damage. In some cases these might be obvious, but others can be very difficult to tell apart. For example, if leaves are brown and crisp, it could be due to hopperburn, caused by a small sucking insect, or to early blight, caused by a fungus – or perhaps another disease altogether.
In 2016, we are redesigning our evaluation sheets so that you can simply comment on the level of damage to leaves and stems, without having to make a diagnosis on the spot. We’ll be sure to provide enough space for comments – and if you can snap a picture or two, that is always helpful.
In steps 5-8, below, we walk you through some of the more common insect pests and diseases, since it’s always helpful to know more about what’s out in the field. Symptoms can vary so use these resources as guides, but be aware that diagnosis can be difficult.
Step 5: How not to get burned in assessing Hopperburn
The photo shows hopperburn severity. The leaves on the upper left would be rated a 5 (green, healthy and from left to right 4,3,2. All the leaves on the bottom would be rated a 1 – wherein leaves are curled with brown, dead edges. A call/email to Dr. Russ Groves Entomologist, or a look through the UW Vegetable Crop website, can also help demystify leafhopper burn.
Step 6: Colorado Potato Beetle Damage
Ratings should reflect evidence of damage based on overall appearance of the plot.
For example a rating of 1 equates to the most severe damage with only the potato stalks remaining and 5 being no damage from beetles or larvae.
Step 7: The infamous Other – assessing signs for other insect pests
Sometimes there will be mystery insects that feast on your potato plants. For example, this past year, I noticed lots of black striped green bugs as well as little holes in the leaves. When in doubt describe the damage the best you can in the comments section.
A few resources we’ve turned to with identifying insects (foes and friends) are the Vegetable Crop Entomology website run by Dr. Groves and the Vegetable MD Online at Cornell Extension.
Step 8: No more dis-ease in assessing disease issues
There are many disease pressures with growing potatoes. Some are visible during the growing season, and others can be seen on the tubers at harvest. The main diseases you might see are illustrated below.
Late blight, that ferocious sporulating nightmare of a plant disease that caused the 1848 Irish potato famine and fueled massive emigration from Ireland, still sends shivers down the spines of any vegetable grower.
The disease is caused by the fungus-like microorganism Phytophthora infestans, – literally “plant infesting destroyer”. In addition to evaluating for evidence of late blight you can stay up to date on whether late blight is in your area through Blitecast, a resource developed by Dr. Gevens and the UW Vegetable Pathology team.” If you are in doubt and are concerned you may have late blight, send a leaf sample to Dr. Gevens.
Potato Virus Y
and Common Scab
In general, a rating of 1, means the plants are wiped out by disease and a rating of 5 means no evidence of disease damage. Again, don’t get too hung up on identifying the disease. If you are curious, you can send leaf samples to UW Plant Disease Diagnostic Clinic. There are also lots of great resources and photos on plant diseases in the UW Vegetable Pathology website, including up to date information on late blight forecasting.
Above all, don’t get too concerned about the exact rating descriptions. Take some time to size up your potato plants and notice the different varieties in relation to each other before you start to rate them. Are there major differences that stand out? The ratings are just a way to show which varieties were better or worse than others.
In all cases, don’t forget to celebrate your efforts and taste the results. Tasting feasts are a great way to get feedback from your friends, CSA members, market customers, and neighbors.
After wandering the virtual fields from tuber to table, let us know your thoughts?
What tips or resources do you need to help you better assess potato varieties during the growing season?
We’d love to hear from you as we make plans for field research and thanks for your feedback, participation.
This article was posted in Blog Posts and tagged Colorado potato beetle, Late blight, organic potato, organic weed management, potato leafhopper, potato variety trials, variety trials.