Would you like your tomatoes grown with less pesticides and fertilizer?
In recent years, tomato producers have seen a dramatic decrease in the yields of their crops. There are several intertwining reasons for this drop in production and finding a solution involves a delicate balancing act. We intend to explore possible solutions that will not only benefit tomato producers, but the consumer and possibly the environment as well.
Whiteflies are one of the leading causes of tomato yield loss worldwide, whether in greenhouse or agricultural field operations. Because they suck sap rich in sugars and amino-acids, whiteflies cause general weakening and reduced growth of their host plants. In addition, whiteflies are vectors of several kinds of viruses, which cause diseases that often kill the plants.
In the last four years, Canada has produced, on average, 410,862 metric tonnes of tomato, which represent a farm gate value of approximately $70 million dollars. In this same period, production has fallen by almost 40%, and while there are many factors to explain these changes including a decrease of 28% in area planted, the use of tomato varieties that depend much less on the application of pesticides would benefit farmers and consumers alike.
Plants may defend themselves from herbivores by means of resistance traits (those that reduce the amount of tissue removed by herbivores), or tolerance traits (mechanisms that reduce the detrimental effects of the loss of tissue on plant fitness). Resistance traits include trichomes (hairs), thicker or tougher cuticles and cell walls, thorns, and a variety of chemical compounds with toxic or anti-digestive properties, including volatiles that attract the natural enemies of herbivores. These traits can be more or less effective at reducing the frequency of attacks from herbivores. Making and maintaining these structures and compounds represents an important cost to plants. Traits that contribute to tolerance include meristem availability and the capacity to store and mobilize resources such as carbohydrates and nitrogenous compounds.
Tomatoes with mould as a result of whiteflies (photo credit: Nigel Cattlin)
Plants obtain the materials and energy needed for metabolism, growth, and defence through photosynthesis and nutrient uptake. Soil nitrogen is commonly insufficient for adequate crop growth. For this reason, farmers supplement soils with nitrogen-containing fertilizers. While an adequate supply of nitrogen and other nutrients is necessary for growth and fruit production, nitrogen in plant tissues is itself a commodity that herbivores seek avidly. Therefore, there is a level of fertilizer addition beyond which the losses in fruit production caused by greater attraction and feeding of pests, exceed the gains brought about by nitrogen availability. In addition, excess fertilizer will eventually be washed into streams and rivers, contributing to eutrophication and expansion of dead zones in lakes and coastal areas.
Preference for foliage with high nitrogen content has been shown for a variety of chewing insects and sap-suckers, including whiteflies. However, the actual decrease in fruit production due to reduced fertilization has not been tested for different varieties with different levels of natural resistance to whitefly. Moreover, the capacity to recover from whitefly attack under different levels of fertilization in these varieties has not been assessed.
This study is aimed at testing whether soil nitrogen supplementation results in greater susceptibility of tomato plants to whitefly infestation, and if so, to what extent; and testing whether some varieties of tomato are more resistant or tolerant to whiteflies.
We will conduct an experiment with 90 tomato plants from each of four varieties growing under three levels of soil nitrogen fertilization (no addition, half and full amount of the minimum nitrogen addition recommended) in a greenhouse at The University of Manitoba, and we will infest half of these plants with 50 whiteflies after approximately 8 weeks of growth (halfway point to fruit production; total of 360 plants). The whitefly population of half of the infested plants will be kept between 200 and 300 adults. Populations of the other half of the infested plants will be left to grow without our intervention. At about 12 weeks, we will count and measure the number of tomatoes produced by plants.
Different varieties of tomatoes
To test whether whiteflies are more attracted to foliage richer in nitrogen, standard size disks will be taken from the leaves of a separate set of un-infested plants subjected to the same fertilization treatments as above, for preference assays using a simple choice arena. Samples of these leaves will be sent for elemental analysis.
Through this experiment, we hope to find a combination of these factors that will reduce the negative impact whiteflies are having on yields and reduce the need for pesticide application. In turn, this will hopefully make tomato production a more economically viable option for our farmers while possibly even improving the end product for consumers.
This project also constitutes one of the main components of M.Sc. student, Sreedevi Ramachandran’s studies. She will undergo intensive training on plant growth and care, preparation of fertilizer solutions, calculation of fertilizer application rates, techniques for care and propagation of whiteflies, leaf area analysis, use of stereo microscope, training in experimental design and analysis of data through the use of specialized software, discussion of primary literature, communication and presentation of results in oral and poster formats. Sreedevi is being supported through internal grants and scholarships from UWinnipeg.
(Project team members from left to right: Dane Mymko, third-year undergraduate student; Sreedevi Ramachandran, M.Sc. student; Germán Avila-Sakar, Associate Professor)
The following are the costs and materials needed for this project:
- pots and trays - $260
- peat moss-based media - $140
- fertilizer - $50
- leaf nitrogen content analyses - $150
- anti-insect screen - $390
Total project cost = $990
**Greenhouse space rental fee is being waved thanks to collaboration with colleagues from the Department of Biological Sciences at The University of Manitoba.
If you don’t want to make a gift online, you can also send a cheque to:
The University of Winnipeg Foundation
901 - 491 Portage Avenue
Winnipeg, Manitoba R3B 2E4 Canada
*Note: Please make sure to mark your cheque for "SUSTAINABLY SAVING TOMATOES" and your gift will be counted towards this initiative!
Call toll-free: 1-866-394-6050
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