Ok let’s talk about RUST!

May 5, 2025

Ok let’s talk about RUST. Rust is a fungus from one or more closely related families. Puccinia sounds like an Italian mushroom, but it’s not. Puccinia arachidic (peanut rust), graminis (stem rust), heucherae (heuchera rust) are the 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.

Rust grows and spreads quickly. Its ideal conditions are temperatures between 50F/10C and 68F/20C with humidity above 40% so a rainy spring/Fall Day. That is why here in the PNW we only see it pop 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, BINGO, it’s now on the leaf. Over the next 8 to 24 hours the filament of the spore is spreading into the cell walls of the leaf. Twenty-four to thirty-six hours later you will see the fruiting body (an orange pustule) on the leaf kind of like a mushroom. Once you see the fruiting body 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 75F/24C for a week or so rust will slow down or stop all together 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.

There are many products out there to treat rust. But right along with treatment using a chemical spray is sanitation. We here at TERRA NOVA® will remove or cut back the leaves the day or so after we spray. At TERRA NOVA®, we put them in the dumpster not the compost pile. Try not to handle heathy plants after or during a rust clean up. Use gloves and wash your hands between cleaning up and your next project. I hope this has been helpful.