Asked June 12, 2020, 4:06 PM EDT
I have noticed "rust" on the leaves of some flowering plants. Insecticidal soap doesn't help. What do you suggest? Also my swamp milkweed lower leaves are yellowing and falling off. Thank you.
Cumberland County Pennsylvania
Thank you for your questions.
Is this the first time you are seeing rust in your garden? It is a fungal disease so insecticidal soap will not work.
You didn’t specify which plants are infected. Knowing which are affected could help fine tune our recommendations. Rust fungi have complicated lifecycles that include five different types of spores and sometimes two completely different host plants. Each species of fungi is only able to infect a limited group of plants. Some infect only one species of plant. Others are able to infect several plants within the same family. Some rust fungi have an “alternate host”. These rust fungi spend half of their life on one group of plants and the other half of their life on a completely different group of plants. Other rust fungi have no alternate host and their entire life cycle occurs on one plant or plant family.
Spores of rust fungi are easily spread by the wind and occasionally splashing water. They need a film of water on the foliage from rain, dew, or irrigation to germinate and start a new infection. Rust fungi can be introduced to the garden on infected plants or as windblown spores from nearby infected plants.
Helpful control measures include pinching off rust infected leaves and disposing of them in the trash, not compost. Do not remove more than 1/3 of the plant’s foliage at any one time. Reduce moisture on leaves by watering at the base of the plant and early enough in the day to allow the plant to dry.
Space plants far enough apart to allow air movement between them. Inspect your plants regularly. Examine the inner and lower leaves where humidity is high.
Fungicides are rarely necessary to manage rust in a home garden. It is ok to tolerate low levels of rust in the flower garden. A little spotting on the leaves rarely does significant damage to the health of the plant. Hand removal of infected material is your best option if the infection isn’t severe.
If a fungicide is to be used, it must be applied to healthy green tissue early in the growing season before infection begins. Examine the lower leaves of plants with history of severe rust once a week. When the first leaf spot is observed, pinch off the infected leaves and begin fungicide sprays to protect healthy tissues. Repeat applications are often necessary throughout the growing season and should be applied according to label instructions. Fungicides will not cure or remove existing rust infections. Once the majority of leaves have leaf spots, it is too late to treat.
Fungicides with the active ingredient Myclobutanil or Azoxystrobin are most effective against rust fungi. Fungicides with the active ingredient Chlorothalonil or Thiophanate methyl provide some protection against rust fungi but not complete protection.
An infection from the previous year can persist from one season to the next in plant debris, setting up infection for the following year. Cut down and remove any infected plant material at the end of the growing season.
Below is a useful fact sheet for understanding rust and how to treat it.
Could you provide a little more information on your milkweed? Are these established plants? Is this first year you are seeing the yellowing/dropping lower leaves? Depending on the situation, a few causes could be considered.
Lower /older leaves or those that do not receive enough sunlight can drop off. Too much or too little moisture or nitrogen could also be a problem. How does the rest of the plant look?
There is also a bacterial disease called Milkweed Yellows. It is spread by leafhoppers. In this situation the leaves are curled and or twisted. I am finding no reliable information on this condition at this time but will investigate further if you have these distortions.