by Sally Colby
Emptying a greenhouse is just the start of a thorough disinfection process. Cleaning and sanitizing prior to the next crop is essential, but managing crops to prevent disease is also vital for greenhouse cleanliness.
Dr. Ann Chase, Chase Agricultural Consulting, said an integrated disease management program that includes consistent and thorough sanitation is the first line of defense against disease, and growers who ignore sanitation face a losing battle. “It can’t be hit and miss,” she said. “Every single person in your operation has to understand why they’re doing these things – it has to be uniform.”
The best sanitation programs involve a combination of chemical and physical measures. “If something is unsaleable, it needs to be thrown out, not stacked up and left on the bench,” said Chase. “If you have dump containers or a five-gallon bucket full of diseased leaves, get them out of the greenhouse. Don’t just collect the stuff, take it out.” Chase added that bagging waste or diseased crops is a good practice to help stop the spread of diseases such as downy mildew.
When possible, use reusable flats, but they must be clean with no potting media left in any cells. If flats with bits of potting media are dunked into disinfectant, they won’t be properly disinfected.
Chase explained that fusarium and Thielaviopsis are soil-borne and can survive for a long time without a host plant. Without sanitation, these and other diseases will continue to devastate a crop, but cleaning and sanitation can help manage the problem. Algae is a common greenhouse issue, and Chase said the same conditions that allow tropical plants to thrive are also perfect for algae.
Sanitation at propagation is the basis for a true IPM program. “If you aren’t able to do some of the things to have good sanitation and good methods of propagation,” said Chase, “it doesn’t matter how many fungicides and bactericides you buy – you will lose money.”
Propagation presents numerous problems in the greenhouse, and Chase said it’s because cuttings and seeds can harbor pathogens. “The characteristics of propagation lead to disease,” she said. “There’s a lot of water, very high humidity and poor air movement.” Chase added that handling causes wounds, which provide entry points for diseases such as Botrytis.
Chase advised growers to not reuse water in propagation, but if it’s necessary to reuse water, it should be treated. Chase reported having consistently good algae control with the use of ZeroTol®.
Regarding the use of chemicals in propagation, Chase said, “If you’re trying to put roots on an unrooted cutting, you may stop disease but if you stop roots from forming, you have no crop. You have to pay attention to more than stopping the disease.”
Some bacteria and fungi are easily and routinely seed-borne, including Alternaria, Pseudomonas and Xanthomonas. If seedlings emerge with spots on the cotyledons, there’s a good chance seed-borne disease is present. “Treating these at any point from now on will be a catch-up game,” she said. “For the last 20 years we have been pulling plants from all over the world and they get to us overnight. That means all the problems in South Africa, Costa Rica and Brazil are getting here immediately. This is an issue because there’s no such thing as ‘we don’t have it here.’ We do have it here, and it’s coming in just about daily.”
When Chase started to look more closely at incoming shipments of cuttings, she found unrooted Sutera cuttings that arrived with obvious Pythium, which had to originate at the farm of origin. She also found Pseudomonas on salvia, downy mildew on coleus, and Pythium on the base of unrooted geranium cuttings.
Chase has talked with many poinsettia growers over the years about why propagation is such a problem, and it’s the water. “A lot of people say they can grow poinsettias well without any Erwinia problems if they stay on top of the amount of water,” she said. “They stop the overnight watering as fast as possible. If you don’t, you get Erwinia, and I can tell you that in my experience in trying to stop Erwinia, it is hard to stop. It isn’t just something you can spray to stop.”
For many diseases, water management is the first line of defense. Problems are usually the result of too much water or water at the wrong time of day. Chase uses the example of Rhizoctonia, which in the presence of a lot of heat moves quickly. A tray with healthy seedlings at one end and diseased seedlings at the other end will quickly turn into an entire flat that’s contaminated. If there’s any evidence of disease starting, Chase advised growers to cut losses early and toss out the entire flat. Avoid the temptation to rescue part of a tray because plants that appear healthy will likely become infected. If obviously infected leaves or cuttings are simply placed in a trash pile in the greenhouse, they’ll continue to spread disease. Prompt removal of diseased tissue from the greenhouse is the only way to prevent further disease.
Propagation requires mist and/or water, both of which are potential catalysts for disease. “In propagation, you can use something like Reemay to keep the humidity up without getting the leaves very wet and get roots without wet leaves,” said Chase. “You’re going to have a much better time controlling diseases.” Another option is to use layers of newspaper over cuttings to help maintain humidity without wetting the leaves.
In addition to managing water, Chase emphasized the importance of not using contaminated seeds or cuttings. If seeds or cuttings arrive with disease, be aware of what you’re dealing with and treat immediately to avoid ongoing problems.
She cautioned growers to be careful about using fungicides or certain water treatments on unrooted cuttings. “You can cause problems with water disinfectants and some fungicides,” she said. “If you recycle water, treat it; and don’t use recycled water in propagation. You’re setting up your whole nursery for disaster if you recycle water.”
Managing water involves both timing and amount. “The amount of water, when you water and the way you water is critical,” said Chase. “Get plants out of propagation as fast as possible. There’s a lot of data showing that RootShield® Plus, Heritage®, Mural® or Pageant® Intrinsic® can shorten the time in propagation.” She added that such products help control disease and provide growth benefits.
Chase reiterated the importance of not trying to rescue portions of trays with diseased plants. “Don’t try to cure trash,” said Chase. “Trash is trash. Throw things away when they should be thrown away.”