Page 18 - Gazette - November 2023

COOLER WEATHER IMPACTS MOISTURE MANAGEMENT

Relative humidity affects plant transpiration and gas exchange, and allows for plants to
photosynthesize, so managing the moisture content in the greenhouse is essential to maintain
plant health. As the temperature falls, air can hold less and less moisture, so cold air will have
a higher relative humidity than warm air at the same absolute humidity. Higher relative humidity
environments coupled with cloudy cool weather reduces both transpiration and growing media
dry down rates. Media staying saturated for extended periods of time can lead to root borne
pathogens like Pythium. The use of radiant heat in the floor or bottom heat on the bench will also
help dry out the media and help prevent root rot diseases. Segway O (MOA 21), Fenstop (11),
Banrot (I/14), Truban (14), Terrazole (14) and biological-based fungicides such as Cease (BM02),
Stargus (BM02), Triathlon BA (BM02), RootShield Plus WP/G (BM02), and Obtego (BM02) are
all effective options for Pythium management. See Figure 1 for an example of roots affected by
Pythium.

THE RELATIONSHIP BETWEEN RELATIVE HUMIDITY,
BOTRYTIS, AND CALCIUM
Relative humidity over 94% and continuous leaf wetness for 12 hours promotes Botrytis spore
germination, however, Botrytis can begin sporulating within as quickly as 2-3 hours under the
right conditions! This problem might sneak up on growers who do not take proactive measures to
control latent conditions, which might not be recognizable as an issue until it's too late.

Culturally, air movement with HAF fans, proper plant spacing, and irrigation practices that allow
foliage to dry before nightfall are important tactics used to reduce condensation, and thus,
Botrytis. Maintaining nightly greenhouse temperatures above 60 degrees F will reduce the
possibility of condensation forming on leaf surfaces. Removing and disposing of diseased plant
material properly from the greenhouse is key in eradicating the spread of diseases. To do this,
scouting will be necessary to identify the first signs of damage. Figure 2 shows early Botrytis
damage to poinsettia leaves.

With cooler growing conditions, plant transpiration and overall nutrient uptake rates decrease.
Calcium is a secondary macronutrient that relies on plant transpiration to be translocated to newly
developing foliage and flowers/bracts. Calcium chloride dihydrate can be foliar applied (e.g., 0.5
oz per 3 gal or 1 lb per 100 gal) with CapSil (1/2 tsp per gal or 8 oz per 100 gal; ¼ tsp per gal
or 4 oz per 100 gal on poinsettia bracts) to address calcium deficiencies, help strengthen plants
to hold up in shipping, as well as provide additional support in preventing Botrytis infections.
Even with these cultural methods in place, Botrytis can still be a resilient pathogen. Proper
rotation between effective fungicides such as Affirm (MOA 19), Astun (7), Postiva (3/7), Mural
(7/11), Pageant (7/11), Medallion (12), Palladium (9/12), and BotryStop (NC) are important from a
preventative and curative standpoint.

18 | GRIFFIN GAZETTE 2023