Start Scouting your Potato Fields

Tianna DuPont, Penn State Extension; Dr. Beth Gugino, Penn State Plant Pathology; Dr. Shelby Fliesher, Penn State Entomology

Late Blight

Phytophthora infestans (aka the disease that caused the Irish potato famine) has not been found in PA yet this year. But it was confirmed on potato in New Jersey last week and on Long Island, NY this week. In both cases infected seed are thought to be the source of the pathogen. Infected volunteer potatoes that survived the very mild winter are also a potential source. Because this disease can devastate a planting in a week, and can spread quickly to neighboring fields; it is critical to make sure to scout for symptoms and take preventative precautions.

Late blight forms greasy, water soaked lesions that become brown or black in a few days. They are often surrounded by a halo of light green tissue. Under high humidity you can see delicate white mold (sporulation) on the underside of the leaf.

For additional photos go here.

If late blight is present in your county or surrounding counties it is recommended that you spray preventatively even though it is not in your field especially when the conditions are favorable for disease (cooler temperatures 65 to 70°F and high relative humidity or leaf wetness). For organic growers copper-based fungicides containing the highest amount of copper hydroxide are most effective (Champ WG and Nu-Cop 50DF*). For conventional growers check your state’s Commercial Vegetable Recommendations Guide. For additional information on preventative practices and organic management see the Organic Potato Production Guide.

Early Blight

is generally not a problem until early to mid- July with warm humid weather and with the on-set of fruiting. But the warm and wet conditions that have prevailed this spring have been favorable for early blight development.

Early Blight on Potato

This fungus initially causes lesions or spots on the lower leaves that are small and dark brown but as they expand faint concentric rings develop. If it is warm and wet, the lesions may coalesce until the leaf looks all yellow and brown. Severe defoliation will reduce yields.
There is not an established threshold for early blight. Cultural controls include a minimum two- year rotation (allows the crop residue to thoroughly decompose), resistant cultivars, minimizing nutrient and drought stress, and selection of well- drained fields far from other hosts. For additional information see organic/ commercial guides above.

Colorado Potato Beetle (CPB)

Adults have alternating black and yellow stripes that run lengthwise on the wing covers. Learn to identify the eggs that are bright orange and deposited on the underside of the leaf in clusters of 20-40 eggs. Larvae are small and red with two rows of black spots on their backs (make sure you can tell the difference from lady beetle larvae).

Colorado Potato Beetle Larvae

This is a serious pest of potato. It can reduce yield up to 90%.

Encouraging natural enemies, at least one year rotation to a non-host crop, resistant varieties (Elba, King Harry, Prince Harry) and avoid planting close to last year’s potato fields with high CPB levels are important cultural controls. 

For organic growers picking larvae up to two acres can be economically viable. Entrust has the highest efficacy (14 of 14 trials) on larvae and eggs, but should be rotated with cultural controls (flaming, trap cropping, vacuuming) or alternative organic insecticides.* For resistance management do not use on both generations of larvae. For conventional growers see the Commercial Vegetable Production Recommendations. Mulch and trench traps are additional controls to consider. 

Colorado Potato Beetle Eggs

Colorado Potato Beetle

Potato Leafhopper

Tiny, iridescent green, wedge shaped leafhoppers use their needle-like mouth parts to probe in the plant tissue.  Their feeding leaves behind sheaths that are formed from the insect’s saliva, and their saliva also causes plant vascular cells to enlarge (hypertrophy).  Soon the phloem is blocked, which keeps the plant from being able to more water and nutrients to those cells and they die. The ‘burn’ you see is dead tissue.

Hopper Burn from potato leafhoppers.

Potato leafhoppers are a risk every growing season. Short of late blight they are the most serious pest of potato. In the worst case, left uncontrolled, you may not harvest more than you planted. 

It is difficult to predict when leaf hoppers will be present without monitoring. Sweeping the field or yellow sticky traps are good ways to see if you have reached the threshold.  Also be aware that this insect has many host plants.  Populations are often highest in alfalfa, and when alfalfa is harvested, adults will fly in search of new hosts.

Resistant varieties such as Elba and King Harry have hairs that deter the leaf hopper from feeding. Row covers can be used to exclude leafhoppers early in the season. For organic growers Neem and Pyrethrins have some efficacy, but trials are limited.

Flea Beetles

These tiny, shiny beetles have enlarged hind femurs (the first leg segment) that enables them to jump when they are disturbed.  They chew tiny holes in foliage. The adult beetles rarely cause yield reductions but high levels of larvae in soil can cause tuber damage.

Flea Beetle on Potato

*Always check with your certifier to make sure products are OMRI labeled and allowable.

Resources:

Organic Potato Production Guide
Commercial Vegetable Production Guide
Organic Late Blight Management Penn State Extension
Organic Late Blight Management of Potato and Tomato eXtension