Don’t waste your money on prevention

Did this title grab your attention? If it didn’t, I’d love to scout your greenhouse operation and see if you have pests and diseases. Unless you’re a plant pathologist or entomologist actively looking for bugs and diseases to use in their next research experiment, you’re buying plants because they are beautiful and healthy. Quality plants are usually pest and disease free.

I tease my entomologist friends by saying that insects are easy — you see the bugs and you spray them. I only use that joke as a comparison to plant pathogens that cause disease. Most plant pathogens are microscopic and can’t be seen with the naked eye.

However, you can see heavy sporulation of pathogens like Botrytis, downy mildew, powdery mildew and rusts on the leaves of plants. The growth and sporulation of a pathogen on a single leaf spot often consists of tens of thousands of spores, which are microscopic individually.

In fact, a single spore of Botrytis is roughly 10 microns in diameter, which makes it about 100 times smaller than a millimeter. One can quickly understand how easily these microscopic spores can spread and go unnoticed in a greenhouse.

Culture considerations

Many greenhouse growers are masters at maximizing the use of space to accommodate as many plants as possible in a confined area. But the same confined area of plants that maximizes production in a greenhouse also serves as a feeding and breeding ground for pests and pathogens. This makes it difficult to produce high-quality plants. Air flow that allows for the plant canopy to dry between irrigation events is crucial for disease management. Spacing plants to allow for adequate drying of the foliage minimizes the opportunity for spores to germinate and infect plants. Thus, spacing plants is a cost-effective means of controlling disease outbreaks.

Routine scouting is important for reducing the potential of pest and disease outbreaks, as well as monitoring and assessing biological control and quality. Once a plant is damaged or a population of beneficial insects falls, there’s no going back. Regular scouting helps to identify issues early and allows for quick action control efforts that are usually less costly and drastic. In addition to scouting, growers need to have access to a diagnostic lab for the identification of new pests and diseases. Always keep accurate records, as new pests and diseases will likely become recurring issues.

Plant medicine

Pesticides play a vital role in preventing and controlling plant pests and diseases. They are the medicine that helps keep our plants healthy and salable. Since we don’t always have ideal growing conditions, use of pesticides becomes an important part of our plant production schedule.

Some growers are reluctant to make changes or apply anything new, and rightfully so. Others are fine with trying every new product that comes on the market. Regardless of your attitude toward new products, the potential for phytotoxicity makes reading and understanding labels critically important. Most pesticide manufacturers (usually not as true for generics) have completed extensive trials, evaluating the product on ornamental plants and include detailed instructions on how best to apply the product. This includes restrictions. If the label doesn’t address your application needs, then it’s highly recommended to conduct a trial under your specific conditions before application.

The pesticide label is the law, so it’s always worth investing the extra time to review the product label to ensure it can be applied legally and that it addresses the pest or disease in question. Application rates and intervals should be strictly followed, as they have been evaluated and proven effective through extensive research conducted by the manufacturer and university researchers. Application timing is also critical, as some products may not provide knockdown or curative activity and will only be effective when applied preventatively. If you have trouble interpreting a pesticide label, ask for help. University extension agents, technical staff with companies, and private consultants are available and should be consulted if there is a question or doubt before applying a product to the crop.

Important questions and concerns surrounding the use of pesticides focus on product efficacy and expectations for control and plant safety. Research has shown that both rotation and tank-mixing are effective means of limiting pest and pathogen resistance development. The convenience and labor savings of using tank-mixes may be attractive for growers, but you cannot decrease the rate by putting two products in a tank.

Be sure to follow the labeled rates and ensure there are no issues with product compatibility or plant safety. Pay careful attention when measuring pesticide quantities and the specific order of products added into the tank. It’s possible that the efficacy of a product could be diminished when tank-mixed with other products, so be sure to closely evaluate the performance of the product alone first.

Most pesticides, especially drench applications, work best when applied preventatively or when pest pressure is low. Systemic pesticides are taken up by the roots and move through the vascular tissues of the plant. Depending on environmental conditions and the product being used, this can take hours or days before it protects the plants or kills pests and pathogens.

Products with translaminar activity that move from the upper leaf to the lower leaf surface can also take time. Applying before pests and pathogens are present is important. When you see pests, you should assume they are actively feeding and causing damage to the plants. Whether it’s mechanical damage from insects chewing or stunting, or distorted growth from piercing and sucking, the damage has most likely already been done.

Once the plant is damaged, all the affected tissue must be removed and discarded, as it reduces plant quality. These losses add up quickly, but they can be avoided with preventative measures in place.