Insights into the initiation of myrtle rust infection

Have you ever noticed that myrtle rust symptoms appear on the newly emerged parts of plants, such as the new leaves, fruits, or flowers? Behind this small detail lies a big clue about how myrtle rust spores are able to enter the host tissues.

So how does myrtle rust infect plants? What are the mechanisms that allow this fungus to penetrate the host tissues and cause disease? In this post, we will briefly explore the initiation of the infection process of myrtle rust.

Myrtle rust disease on cultivated Pilidiostigma glabrum captured by @tonyrddd

The infection process of myrtle rust begins with the spores. In our previous journal post I introduced these tiny reproductive structures of the fungus which are dispersed by wind, water, insects, animals, and human activities. Given the right environmental conditions, these spores “wake up” in a process called germination. This process requires dark and moist conditions along with mild temperatures (between 15°C and 25°C). These conditions are often met during the night, when dew forms on the plant surfaces, or during periods of rain or irrigation.

During germination, a thin threadlike outgrowth, termed a germ tube, forms from the spore and grows towards the plant surface. The germ tube then forms a structure called an appressorium, which is a flattened and swollen tip that adheres to the plant surface. For most rust fungi, this appressorium seeks out openings in the plant surface to enter the plant. Unlike most other rust fungi, myrtle rust goes in guns blazing, taking a more aggressive approach to entering the plant.

After the formation of the appressorium, the tip of which is termed the penetration peg, exerts an enormous amount of pressure that is able to pierce directly through the plant surface to reach the nutrient containing cells under the surface. This mechanism is usual for rusts, but has been observed and studied in soybean rust.

The fungus relies on and recognises the physical traits of the plant surface to initiate germination such as texture and chemical components. There are however many properties of the plant surface which protect the plants from invasion such as the presence of waxes, cuticles, hairs, or glands that can prevent the attachment or penetration of the fungal spores. In the case of leaves as they mature, these wax layers increase in thickness and complexity. This change reduces the ability of the fungus to recognise the plant leading to reduced germination, while also making penetration of any germinated spores challenging due to this increased thickness.

As a result of this unique penetration mechanism, myrtle rust spores are generally only observed on the new tissues of susceptible plants as they are less hardy and provide the right physical cues for spores to germinate. In the case of resistant plants which show no signs of myrtle rust infection, some scientists are working to identify if there are changes to the leaf structure and chemical composition that prevent myrtle rust infection even in young leaves.

If you see a healthy plant in a myrtle rust impacted area, please upload it to iNaturalist as this may give scientists clues as to what makes these plants resistant to the fungus.

Next time you spot myrtle rust on a hike, in your neighbourhood, or in your garden, be sure to take note of which parts of the plant are infected. You may even be able to spot the tell-tale signs of previous infection that may remain on older plant parts.

In a future post we'll dive into what is happening once fungus has infiltrated under the leaf surface. Until then, if you have any burning questions about myrtle rust, throw them in the comments below and we can answer them in the next journal post!

Thank you all for your contributions.

Alyssa

Posted on February 23, 2024 12:49 AM by myrtle_rust_martino