Pests & Diseases

February 2023 Pests & Diseases

Image from Twitter

Bananas: Expiration Date in Sight

Writing in the Laidback Gardener, biologist Audrey Martel warns of the demise of the Cavendish banana. She notes that artificial banana flavour tastes unlike the bananas we eat today because it was modelled on a variety that has now almost disappeared, the Gros Michel banana from Martinique. Sugars in the Gros Michel were so intense they formed a honey-like syrup. Today’s Cavendish banana was developed by William Cavendish and Joseph Paxton in 1835. Wild bananas, of which there are about 200 species, are often inedible, containing many hard seeds and very little flesh. Wild bananas reproduce sexually, which means more genetic mixing and diversity. The Cavendish banana is reproduced by cloning so each plant is an exact genetic replica of every other plant. This lack of genetic diversity was good for stability of what became an important agricultural crop, but bad in terms of that crop’s resilience to pests and diseases. At the end of the 20th century, the fungus Fusarium oxysporum attacked Gros Michel bananas, killing almost all the banana trees. A few crops in isolated areas were saved, but the Cavendish, more resistant to the fungus, replaced it in world production. Now, however, another strain of Fusarium called TR4 that can kill many varieties, including the Cavendish, is spreading to more and more countries. In many places bananas are a staple crop, so this fungus is a threat to food security as well as livelihoods. A team of scientists has found that exposing plants to another variant of Fusarium provides protection for up to 10 days, but this solution may not be commercially viable, or permanent. We may have to go back to wild variants to breed a new banana.

Researchers from AU Flakkebjerg have studied how plants can keep pests, diseases and fungi at bay by secreting certain substances. This could be used as a plant defense of the future. Credit: Enoch Narh Kudjordjie.

Plant protection of the future may come from the plants themselves

In happier news, researchers from AU Flakkebjerg, Denmark, have studied how plants with different resistance traits interact with their microbes to respond to pathogen attack. The research is published in the journal Microbiology Spectrum. To begin with, the researchers infected two-week-old Arabidopsis genotypes growing in field soil in a greenhouse with the fungal pathogen Fusarium oxysporum, the same culprit that threatens bananas. Then they studied the results. Enoch Kudjordjie explains, “… we were absolutely sure that the plants were actually infected. The qPCR test showed a clear difference between the two genotypes, with the resistant genotype having a much lower level of the pathogen than the susceptible one.” They then continued to explore the differences between the chemistry and microbiomes in the two genotypes, and found large differences. The plant metabolites and hormones studied were distinct in both the healthy and diseased plants, confirming the involvement of certain plant chemical molecules in mediating plant defense. Likewise, they found that microbial composition, as well as microbial community networks, were distinct in healthy and diseased resistant and susceptible plants. Moreover, beneficial bacteria such as the genera Pseudomonas and Rhizobium were mostly enriched in the rhizosphere of infected plants, suggesting an active recruitment of microbes to resist pathogen invasion. This work has deepened our understanding of how plants defend themselves against a fungal pathogen. More importantly, we found a strong and unique association between individual defense metabolites and specific microbes in the healthy and diseased plants of the different genotypes. “These results strongly confirmed that three underlying host components (genes, metabolites and microbiomes), interactively control the plant defense,” explains Kudjordjie. They also suggest a future where plants are cultivated with optimized yield and other agronomic and economic gains without the use of synthetic chemicals.

Mosaic Diseases Caused by Potyviruses. Image from University of California.

Copying nature to help plants resist viruses

CRISPR gene editing is opening up exciting new frontiers in crop research. The CRISPR-Cas9 technique makes it possible to modify a region of the plant’s DNA in a targeted and precise manner, allowing desirable traits, such as disease resistance, to be retained while eliminating undesirable traits. An iterative gene-editing strategy was recently studied to help tomatoes develop better resistance to a several plant viruses. The findings are published in the Plant Biotechnology Journal.

Pests & Diseases Pollinators, Molluscs and Other Invertebrates

Natural Pest Control

Replacing pesticides with ants to protect crops: A team of researchers affiliated with several institutions in Brazil, working with one colleague from Spain and another from the U.S., has found evidence that suggests ants can be used as a natural pesticide for a wide variety of crops.

In their paper published in Proceedings of the Royal Society B: Biological Sciences, the group describes how they analyzed studies conducted by researchers across the world to learn more about the possible use of natural pest control options by farmers and what they learned by doing so. Because of concerns about pesticide use, researchers around the world have started looking into the possibility of using natural pesticides.

One such natural approach has involved the use of ants—they leave the crops alone and instead feed on the insects that damage plants. Use of ants to control pests has a long history, citrus growers in China, for example, have been using ants to control pests in fruit trees for centuries. In this new effort, the researchers wondered what other researchers have found when looking into the use of ants as a natural pesticide.

Fifty-two published research papers involved looking into the use of ants as a way to control pests, covering 17 different types of crops. In analyzing the papers, the researchers found that most of the studies had led to discoveries of ants providing a high level of pest control—and in some cases, the ants were even better at it than commercial pesticides. They also found that the ants did their best work when used with crops grown in partial shade, and were the least effective when used with crops that produce honeydew—in such plants, ants tended to farm the insects, such as aphids, in order to provide themselves with the sweet liquid.

The researchers conclude by suggesting that the use of ants to control pests appears to be a sustainable and inexpensive way to control pests on both large and small farms.

Pests & Diseases

Box Tree Moth

Box Tree Moth: A Toronto Master Gardeners Garden Guide: Toronto is the epicentre of a North American infestation of the box tree moth, Cydalima perspectalis (Walker). The arrival of this invasive pest was officially announced by the Canadian Food Inspection Agency in November 2018.

The moth crossed into the US at Niagara in the summer of 2021, so the US is still in the very early stages of responding to the infestation. This pest (originally thought to have come from Asia) has now reached plague proportions. They are attacking (and sometimes killing) our pretty and widely-grown evergreen boxwood plants, Buxus spp.

An infested boxwood plant is disfigured by the box tree moth by the loss of leaves, by webbing spun by the larvae, as well as larval excrements. Larvae feed principally on leaves of the host but may also attack the bark. They seem to prefer boxwood plants that receive partial shade but can also be found in full sun gardens.

This long-ish article includes many links to other helpful resources as well as tips on identifying box tree moths, and treating the damage they do.

[Editor’s note: this primer was prepared by Desre Kramer, who in addition to being a member of Toronto Master Gardeners is also my wonderful sister-in-law.]

Pests & Diseases

Stressed Plants

Plant-nibbling insects may make it cloudier and cooler: Plants can release certain chemicals to shield themselves from high temperatures and potentially communicate with other plants. They also release these chemicals in response to stress, including when insects chomp on their leaves.

Now, in a study published in the Journal of Geophysical Research: Atmospheres, scientists have found that insect-damaged plants could release enough of these molecules, called volatile organic compounds, to locally alter the atmosphere and radiative budget above a forest. Once panicking plants release the compounds into the air, the compounds can oxidize, transforming into organic aerosols. Like aerosols emitted from human activity, these aerosols can theoretically change how clouds form and how much sunlight clouds reflect.

Now for the first time in a global atmospheric model, Holopainen et al. consider the potential influence insect-munched plants can have on aerosol concentrations and clouds. These results suggest that insects eating plants could lead to stronger cooling effects from clouds, as greater aerosol concentrations typically correlate with sending more solar radiation back into space. These localized impacts won’t happen in an instant, but still, climate models could incorporate aerosol emissions from areas with intense insect herbivory to best estimate potential impacts on local atmospheric processes, the authors say.