Understanding How Plant Rust Manifests and How to Spot and Stop It
Creeping Thistle Rust (Puccinia punctiformis). | kazakovmaksim via Adobe Stock
Rust is a fungus from one or more closely related families. Puccinia arachidic (peanut rust), graminis (stem rust), and heucherae (heuchera rust) are some of the most common types. Unrelated, yet frequently confused with Puccinia is Xanthomonas, or bacterial leaf spot. Frequently, these two maladies are seen simultaneously, each weakening the plant, making an infection of either disease more likely.
Let’s Talk About Rust
Rust grows and spreads quickly. Its ideal conditions are temperatures between 50°F/10°C and 68°F/20°C with humidity above 40%, so a rainy spring or autumn Day. That is why here, in the Pacific Northwest, it only pops up for a month or so each year. The spore is spread on the wind and in water drops. As the rain or irrigation water droplets hit the ground and splash up, they land on the leaf.
Over the next 8 to 24 hours, the filament of the spore spreads into the cell walls of the leaf. Then, 24 to 36 hours later, the fruiting body (an orange pustule) will appear on the leaf, kind of like a mushroom. Once the fruiting body is on the bottom of the leaf, it will sporulate soon. This is how and why one plant is clean and clear, and the next plant is covered in rust.
Once the temperature goes above 75°F/24°C for a week or so, the rust will slow down, or stop altogether if the humidity stays low. This is why sanitation is key; if the infected leaves land on the ground near the plant, as soon as conditions are favorable, it’s back again. Some places may only see an outbreak every few years because conditions are not right at the right time, or they may see multiple outbreaks because all the conditions are right.
For additional information on plant rust and additional visual examples, please read the original article found on the Terra Nova Nurseries website.
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