May 25, 2021
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Strawberry Season, Evaporative Cooling, and Food Safety
Strawberry season has come to Maryland, and many growers use irrigation to help moderate blossom and berry temperatures. If temperatures are too high in the “canopy” area, blossom and early fruit development can be arrested. There are a couple of methods using irrigation water to lower temperatures through evaporative cooling.
Evaporative cooling is the phenomenon where water will absorb the heat of an area, and as it does, it evaporates. This results in cooling the plant surface, and the overall temperature of the plant (fruit and flowers) is lowered.
The kind of weather we have enjoyed over the last few days can be damaging to ripening fruit and developing blossoms. Growers are alerted by weather patterns of air temperatures over 85oF, low humidity and winds over 10mph.
When irrigation water is applied, and how it is applied, has to be carefully orchestrated. Growers monitor the temperatures of the blossoms by inserting a digital thermometer into the heart of the blossom. When air temperatures reach 87/88oF or blossom temperatures reach 89/90oF, overhead irrigation is usually run for about 25 minutes. This should drop the blossom temperature down into the upper 60s or low 70s.
Photo Credit, Guy Moore
The cycle of allowing blossom temperatures to rise to the threshold (89/90oF) then cooled down by a 25-minute period of overhead irrigation can be repeated several times during the day, but generally not after 3:30pm. Overhead irrigation can increase the possibility of plant diseases when foliage stays wet going into the evening hours.
Overhead irrigation can increase the risk of human pathogen contamination as well. By now, the initial water testing should have been done on all sources of agricultural water on the farm and water distribution systems checked and in good repair. If deep-well water is being used and Maryland potability standards have been met (Total coliforms 0, Turbidity <10NTU, and Nitrates <10mg/mL), the risk of microbial contamination through the water source is very low. Overhead irrigation will pose little or no risk to the harvestable fruit.
If pond water is to be the source for overhead irrigation, testing for generic E. coli should have been done a few days before overhead irrigation is anticipated. The proposed Produce Safety Rule’s water standards state that any agricultural water directly contacting the edible portion of the crop must have less than 235 colony forming units (CFU) or most probable number (MPN) of generic E.coli per 100 mLs of water sampled for any given sample. Water distribution systems should have been checked and are in good repair. Since this is usually the first water test of the season and the water is being applied close to the time of harvest, low E.coli levels minimize risk.
If overhead irrigation is used 4-5 days before harvest and the weather is sunny, the risk of microbial contamination is reduced through natural die-off due to environmental conditions.
Photo credit, Chris Walsh
Managing blossom and fruit temperatures can also be done through the drip irrigation system. Drip irrigation is inherently less risky as the irrigation water does not touch the edible part of the crop when used as part of a plasticulture system or a conventional matted row system.
Some growers run the drip irrigation for a few hours around sunrise. This freshens the plants and help keep plant temperatures lower. If blossom temperatures rise, the drip irrigation can be run again to keep canopy temperatures and microbial risk low as well.
There are signs available through the MDA Food Quality Assurance Program to encourage good consumer practices while in the market and on the farm. Contact the Food Safety Group for availability.
This 12" x 18" poster comes either as a corrugated poster board or as a magnetic sign and is perfect for Pick-Your-Own operations.
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Reach out to Carol Allen (Southern, Central, and Western Maryland) 240-994-5043, callen12@umd.edu or Angela Ferelli, (Baltimore City, Cecil & Harford Counties and Eastern Shore) 302-353-7159, angfer@umd.edu.
Call in with your questions about food safety for fresh produce.
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Carol D. Allen, Agent Associate and Editor
Chris Walsh, Professor Emeritus and Co-PI
University of Maryland
Plant Science Food Safety Group
callen12@umd.edu
Questions for MDA Food Quality Assurance Program:
Deanna Baldwin, Program Manager
Food Quality Assurance Program
Maryland Department of Agriculture
produce.safety@maryland.gov
Funding for this publication was made possible, in part, by the Food and Drug Administration through grant PAR-16-137. The views expressed in written materials or publications and by speakers and moderators do not necessarily reflect the official policies of the Department of Health & Human Services; nor does any mention of trade names, commercial practices, or organization imply endorsement by the United States Government. University programs, activities, and facilities are available to all without regard to race, color, sex, gender identity or expression, sexual orientation, marital status, age, national origin, political affiliation, physical or mental disability, religion, protected veteran status, genetic information, personal appearance, or any other legally protected class
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