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Clean soil and water keep nursery plants healthy

A plant pathologist describes non-chemical practices for producing clean nursery plants and protecting them from Phytophthera ramorum and other diseases and pests.

EVERETT. Wash. — Nurseries know that healthy plants mean more sales, and a Washington State University plant pathologist recommends protecting them with non-chemical practices.

Best Management Practices for growing clean plants should focus on soil and water, Marianne Elliott said at the recent Focus on Farming Conference.

In her research at the WSU Puyallup Research and Extension Center, Elliott deals with fungi, viruses, bacteria, nematodes and water molds such as Phytophthera, which is one of the most destructive diseases of nursery, forest and food crops worldwide.

The detection of P. ramorum, also called Sudden Oak Death, can trigger quarantines and have serious economic impact. It affects more than 60 plant genera and 100 host species.

P. ramorum is but one of several different strains. Others have been blamed for the potato blight of 1845 (the Great Irish Famine) as well as crops losses in soybeans, cacao, strawberries, cucurbits and ornamentals.

Control is difficult, even by chemical means, so researchers aim to develop resistant plant varieties.

Elliott, though, looks for practices that rely not on pesticides but on keeping soil and water clean, practices that protect the environment and reduce the chance of loss to invasives.

To minimize the risk of contaminating soil, she suggest removing plant debris from production and display areas and making sure pots are not in contact with the ground. Weeds, which can harbor pathogens, should be controlled, and cull piles should be kept separate and downhill from growing areas.

She recommended two ways of sterilizing soil. A combination of both is most effective:

• Solarization involves tilling, raking, watering and covering with clear plastics. This requires four to six weeks of clear skies and high temperatures. The practice can also be used to sterilize pots and potting mix.

• Steam can be used to raise soil temperature to 125 degrees for 50 minutes, varying with moisture content and bulk density.

The water source is also critical. Using surface water increases the risk of contamination, she said. Streams are not sterile, and the water should be tested and treated before being used for irrigation.

This will become even more important under climate change projections. Precipitation cycles are expected to mean more irrigation in summer and more runoff in winter. This will require more water recycling, especially east of the Cascades, Elliott said.

Recirculated water should be treated with a sedimentation pond, filter, chemical or ultraviolet treatment or a flocculation pond, in which a chemical binds to the sediment and makes it drop out.

Irrigation water can be filtered or treated with chlorine, ultraviolet or ozone. The University of California-Davis has studied removal of pathogens by ultraviolet irradiation of a water vortex, she said.

Nursery runoff water should be monitored for pesticide residues, nutrients and pathogens, Elliott said. The runoff can be intercepted in a ditch where plants can take up the contaminants.

Another technique, slow sand filtration, treats water for drinking in Third World countries.



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