Winter 2015: Wind and rain, the grower’s bain

Regional Report Ventura County by Jim Downer

All the media are full of stories about the impending El Niño effects and possible flooding rains in California. Although nobody can accurately predict the occurrence of rain, we are certainly due for an increased rain year based on statistical trends and intervals of heavy and light rain incidents. If heavy rains are in the near future, growers will need to take measures to protect their nurseries from the many and disastrous effects of downpours. Physical displacement of soil as erosion and loss of soil are common during heavy rains. While runoff water should not leave agricultural properties it has to move somewhere when rains come, so ditches should be cleared of weeds and other obstructions to permit efficient flow of water. Before rains start, it is a great time to consider some general upgrades and improvement to outdoor growing grounds.

Since many pathogens are splash inoculated from plant to plant or soil to plant, it is imperative to prevent the development of flooded or puddled ground near growing areas. Now would be a great time to lay down additional gravel under container beds or other outside nursery areas. Keeping containers off soil, either with a gravel or fiber mat and gravel system, is imperative when trying to control Phytophthora in nurseries. Compacted walkways and beds may become saturated this winter and create ideal sporulation conditions for Oomycetes or water molds which may then move in water flows to new areas of the nursery causing infections where never seen before. Consider boardwalks or additional gravel in known low spots and walkways so that workers don’t move infested mud from one part of a nursery to another.

For Phytophthora sensitive crops it may be wise to increase the calcium levels in containers by adding additional gypsum now to reduce sporulation and potential spread of disease. It is also wise to use preventative fungicides such as mefenoxam, fluopicolide and phosphorous acid to increase plant readiness for Phytophthora increases during wet weather.

This is also a great time for woody plant growers to prune any diseased or dead materials from plants ahead of winter rains because many Ascomycete canker fungi that cause disease in woody plants will have inoculum in dead twigs. When rain comes, spores are splashed to new plants and cause infections. Since this is an El Niño year, it is warm, and warm rains are best for disease promotion. Remove inoculum now: cull and remove weak, diseased or dead plants from the nursery ahead of the rains to cut down on disease spread.

With rains often come winds, sometimes at hurricane force as evidenced in Pasadena, California a few years ago. Greenhouse growers need to consider the effects of wind this winter on their operations and possible crop loss from this damage.  Tunnel houses used in berry and other production are most at risk but other greenhouse materials such as polycarbonate sheeting can be detached by wind.  Now is a great time to inspect and repair these structures or apply new sheeting as necessary. Wind can also move woody plants so that they rub against each other causing injury to the main stem if tightly spaced. Trees that are blown over due to high winds can be damaged and devalued.  Spend time now inspecting trellis systems and staking woody plants to minimize damage that may be coming. 

Outdoor nurseries that have planting media storage piles should start now to downsize these piles or provide new tarps in advance of wet weather. Greenhouse operations with media stored outside should ensure that bales are properly covered with new tarps to prevent saturation of the media. Media bales should be stored off the ground on raised pallets to avoid contamination with soil or mud flows.

The challenge of a wet and potentially stormy winter is to envision what excess water can do in your operation and then try to prepare. Flooding conditions create a time of potential pathogen movement and the best protection for plants is to keep them elevated above the mud and keep workers from spreading it with the movement of machinery or foot traffic. Money spent now on this infrastructure will prevent disease loss later in the spring or summer.

Jim Downer
Environmental Horticulture Advisor
UC Cooperative Extension Ventura County
669 County Square Drive, #100
Ventura, CA 93003-5401
(805) 645-1458 phone (805) 645-1474 fax
ajdowner@ucanr.edu