by Michelle Grabowski, Extension Educator | University of Minnesota Presented at the 2015 Minnesota Statewide High Tunnel Conference Feb. 17-18, 2015

1. Managing Crop Diseases in High Tunnels
Upper Midwest Fruit and Vegetable Growers Conference 2015
Michelle Grabowski, UMN Extension Educator
Important Biology about Plant Pathogens
Fungi, bacteria, viruses, nematodes and phytoplasmas can all cause disease in high tunnel
crops. These plant pathogens can be brought into a high tunnel on infected seeds or
transplants. Fungi and bacteria can be blown in on wind currents or carried on contaminated
tools, workers clothing or hands. Viruses and Phytoplasmas are primarily spread by insect
vectors. Fungal and bacterial pathogens can survive from one season to the next on infected
plant debris and in some cases on contaminated equipment like tools or trellises. Weeds
growing within and around the tunnel can serve as a reservoir host for many pathogens.
Start out with healthy seedlings
 Purchase certified disease free seed or transplants whenever possible.
 Look for resistant varieties to diseases that have been a problem in the past.
 Do not save seed from diseased plants.
 If seed health is questionable, use a hot water or chlorine seed treatment to disinfest
seed. http://ohioline.osu.edu/hyg-fact/3000/3085.html
 Monitor transplants for disease symptoms. Identify the problem. Promptly remove and
destroy infected seedlings. Treat remaining seedlings as appropriate for the identified
disease.
 If purchasing transplants, ask about the growing practices used to maintain seedling
health. Inspect all transplants prior to planting. Reject any plants with unusual
discoloration or growth on any plant part.
Resistant varieties
A resistant variety is able to defend itself against a specific pathogen. Resistance may be
complete or partial. Partially resistant varieties are sometimes labeled as ‘tolerant’. Resistance
is specific to a particular pathogen. Identify the pathogen causing disease in the tunnel and
search seed catalogues for resistant varieties. Cornell University also maintains an up to date
list of resistant vegetable varieties on their veg md webpage.
http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm
Grafting
Grafting a susceptible plant to a disease resistant root stock will provide resistance to root
infecting pathogens like Verticillium wilt, Fusarium wilt and nematodes.
Scout plants regularly

2.  Inspect plants every week totwoweeks throughout the growing season. Closely examine
5-10 plants randomly placed throughout the tunnel as well as any plant with obvious
disease symptoms.
 Look at the lower inner leaves of the plant where humidity is high. Examine both sides
of the leaf, fruit and stem of the plant.
 Diagnose any plant problems found. UMN Plant Diagnostic Clinic diagnoses plant
samples for fee. http://pdc.umn.edu
 Learn about the biology of the pathogen causing the problem and the management
options available.
Keep it Clean
 Use only ‘clean’ soil amendments. Compost should have been heated through prior to
use in a tunnel. Be cautious of herbicide residue carry over in compost and manure.
http://www.extension.umn.edu/agriculture/horse/nutrition/harvesting-ditch-hay/
 Clean equipment & tools prior to use in tunnel with 10% household bleach or a
commercial sanitizer.
 Keep tools specific for high tunnel work. Do not bring tools used in field crops into the
tunnel without first properly sanitizing them.
 Do not allow use of tobacco products in the high tunnel. Workers should wash hands
with soap and water after use of tobacco products.
 Remove all weeds from inside the tunnel and immediately around the tunnel.
 Remove infected plants after harvest. Burn or bury the infected plants away from the
tunnel.
Managing Moisture and Humidity in High Tunnels
Fungal and bacterial plant pathogens need high humidity and moisture to infect, reproduce
and spread. Reducing free moisture on leaves and fruit as well as relative humidity can
significantly reduce disease problems in the tunnel.
 Use polyethylene covering with anticondensate properties or apply a wetting agent to
prevent condensation on the plastic and dripping onto the plants.
 Use drip irrigation. Adjust watering to provide adequate water for the plants but nothing
excessive. There should never be puddling in the rows.
 Space rows and plants to provide good air movement through the tunnel. Air in the
plant canopy has the highest humidity in the tunnel. Air movement will reduce this
humidity.
 Control weeds in the tunnel.
 Stake and prune plants to maintain air movement through rows and plant canopies.
 Open side vents, doors, and end walls to allow air movement through the tunnel.
Consider adding vents above doors if end walls do not open completely. Ridge vents can
also be added to high tunnels to improve ventilation.
 Release moist air from tunnel in the evenings.
Reducing Humidity in the Greenhouse
https://extension.umass.edu/floriculture/fact-sheets/reducing-humidity-greenhouse
Rotate Crops
Many plant pathogens infect multiple plants in the same family. For example the fungus that
causes early blight can infect tomatoes, potatoes, eggplant and pepper, but does not infect
cucumber. Many fungal pathogens can survive 2-4 yearsin infected plant debris. Many

3. bacterial pathogens can survive 1-2yearsin crop debris. Growing crops from different families
each year provides time for any surviving pathogens to die out before a new host crop is
present.
A few pathogens can infect plants from many different families or make resting structures that
can survive many years in the soil. Rotation will not be effective in managing these diseases.
For example white mold can infect cabbage, carrots, lettuce and tomatoes and produces resting
structures that survive 8 years in the soil.
In an ideal rotation tomatoes would be planted one year and no related plant would be planted
for 2-4 years. If this is not economically feasible, rotating away from tomatoes for even one
year will be beneficial.
Fungicide Use in High Tunnels
 A high tunnel is legally a ‘Greenhouse’ for pesticide application laws in the state of MN.
 If a pesticide label prohibits use in a greenhouse, it cannot be used in a high tunnel. It is
ok to use a pesticide in a high tunnel if the label allows use in a greenhouse or says
nothing about greenhouse use.
 If there are special instructions on the label for use in greenhouses, these must be
followed in high tunnels.
 Always read and follow all label instructions when applying a pesticide.