Thousands of years ago, plants lived their entire lives with little help from human hands. Times have changed since the advent of the modern-day greenhouse; plants are now at the mercy of the grower. While there is much a grower can do to control the environmental stresses that affect plant health, the unforeseen and unexpected still happens. At these times, it is comforting to know that the survival tactics that are an inherent part of plants’ nature are still in play today. One of the keys to minimizing environmental stresses such as pathogens, insects, drought, cold and heat may lie in strengthening plants’ natural defense systems.
Natural Defense Systems
Plants have developed several strategies to prevent pathogen attacks or hinder their development. Within hours of a pathogen attack, plants try to isolate the pathogen by killing off cells at the infection site, a process known as the hypersensitive response. The hypersensitive response is set in motion by the presence of a pathogen’s “avirulence” genes (Avr genes), which are recognized by disease-resistant genes (R genes) within the plant. If no match is present, the plant is vulnerable to the disease.
Pathogen detection sets off a chain of events. Dying cells release chemical signals that move through the plant via signaling induction pathways. As a result, the plant produces disease-fighting proteins and antimicrobial phytoalexins that help provide plant-wide immunity to the pathogen. The entire process is known as systemic acquired resistance (SAR).
The SAR response is just one way plants can defend themselves. Scientists still have much to learn about plants’ defense systems because they are complex. A variety of activators or signaling molecules can set off chain reactions that trigger different responses to not only pathogens, but also insects, drought, heat and cold. As crop protection companies learn more about how plants defend themselves, they are beginning to develop products that enhance and improve plants’ reactions to environmental stresses.
Triggering Plant Defense Systems With Pyraclostrobin
Another way to help plants’ withstand outside stresses is to conserve the carbohydrates produced in photosynthesis. Pyraclostrobin-based fungicides control a wide variety of diseases from anthracnose to Pythium and Botrytis. Pageant Intrinsic brand fungicide includes this active ingredient. A side effect is that the products also have a plant health benefit. “These products make respiration more efficient,” says Paul Pilon, owner of Perennial Solutions Consulting, which specializes in commercial greenhouse production. “There are more carbohydrates left for other plant functions, like increased nitrate reductase activity and the production of antioxidants. They boost a plant’s health so that it can tolerate some of the stresses of production.”
Pyraclostrobin works as a fungicide by inhibiting mitochondrial respiration and causing the breakdown of the carbons needed for energy to fuel growth. Nitric oxide levels rise because nitrate reductase activity intensifies. This galvanizes plant defense systems into action and decelerates ethylene production. Ethylene is a plant-stress hormone that can cause early maturity, as well as premature flower and leaf drop.
“A lot of what we are seeing in our research is ways that Intrinsic products can be used post-harvest, to help improve cold and drought tolerance and to minimize shipping stress,” says Pilon. During his experiments, Pilon noticed it took treated plants longer to get to a wilting point, and more importantly, they recovered faster than untreated plants. “Although these results are favorable, Pageant is not meant to be used for water conservation. It is not a wetting agent, but this may be a potential benefit growers observe in shipping or at retail locations,” Pilon says.
Pilon also conducted cold tolerance trials where he tried to mimic the freezing conditions plants might encounter in the spring or during winter shipping. “Results can be unpredictable, because it is difficult to reproduce the conditions that occur naturally during frost events,” says Pilon. “What I can say, is for the most part, the treated plants recovered more quickly after stress events and resumed normal growth faster than untreated plants.”
Shipping is a major stressor for plants because of the production of ethylene gas, heat build-up and/or cold injury in the container or truck. Thousands of dollars are lost every year because damaged plants hit the dumpsters upon arrival at the retailers or shortly thereafter.
“We first started using Pageant Intrinsic in June or July, during the heat of the summer,” says Elaine Peiffer, senior grower at Swift Greenhouses in Iowa. “We sprayed it on our perennials before they were shipped out to customers. This year we had very few claims with shipping, and this was a very hot summer for us.”
Treatment with Pageant is also beneficial at the beginning of production. “We use a lower rate of Pageant on our annuals and perennials within the first 24-48 hours after sticking to try to relieve stress, shorten rooting time and increase rooting under mist,” Peiffer says. Pilon has also done some work with unrooted cuttings and transplanting with favorable results.
“For growers who are new to Pageant, I recommend it for use on unrooted cuttings and also in the summers. It is helpful with some of the cool season crops such as poppies and delphiniums for overall heat stress, as well as for shipping,” Peiffer says. “Growers just need to be careful not to overuse it.”
Both Pilon and Peiffer agree that Intrinsic products are not a one-stop solution for every plant health problem, and that overuse can lead to resistance problems. Pilon suggests using them for a targeted problem and then rotating them with other products not in the strobilurin family. “The key is to use Pageant responsibly so we continue to have it as a tool down the road,” Pilon says.
Fungicide First, Plant Health Benefits An Added Bonus
Intrinsic products are the first crop protection products on the ornamentals market to include “plant health” on the label, but they are primarily for fungicides. The plant health benefits they generate are an added bonus, but are not always guaranteed. “I have done enough work with plant trials and research to feel that the benefit is there,” Pilon says. “The magnitude of the response changes from time to time and is based on the conditions, but for the most part the results have been very favorable.”
Pyraclostrobin-based fungicides definitely have potential when it comes to shoring up plant defenses and strengthening plant response to environmental stresses. In the long run, that means better plant health and better disease control, both of which equate to delivering a better product to the end consumer.