There are certain problems that you may find when you have Asclepias (Milkweed) and they are probably the same ones that are common to most of the flowers you may have in your garden.
Many are caused by fungi and some are caused by bacteria. Still others are caused by insects and bugs.
If humidity is high but dry (sounds strange, doesn't it?) then you may end up with Powdery Mildew on the tops of the leaves. Powdery Mildew is this white, powdery-like spore. For example, if you have fog in the morning then sunshine later in the day, then this is an example of 'humid but dry.' Remove the leaves carefully. and water during the day when the leaves will be able to dry. Nothing you can do about the fog!
If your leaves start to get spotted then you may have Leaf Spots. This can be caused by a variety of fungi and usually happens when conditions are wet. It can also be Bacterial Leaf Spot which is definitely a potential problem for Monarchs. This can be caused when plants are water-soaked irregularly. A way to avoid this is by avoiding overhead watering of plants, particularly in the evening, and increase air flow to promote drying of the foliage (leaves). Be sure to remove all leaves that appear diseased and do not use them as food for your caterpillars! Also be sure to pick up any leaves that have fallen to the ground.
Rust causes the Milkweed plant to end up with yellowed leaves and end up stunted in growth. It also causes reddish-coloured spots on the leaves. Remove the leaves carefully so as not to scatter the fungal spores.
There is a virus called the Cucumber Mosaic Virus (CMV) that causes plant foliage to 'crinkle up' and also stunts plant growth. This can attack Milkweed but also mimics other types of damage (spider mite damage, for example).
The Spider Mite is a gnarly little arachnid that is small and nasty to the Milkweed plant. They are another sap-sucker, like the Oleander Aphid, that can cause great deformity to leaves. If your leaves are crinkling, don't assume that you have CMV. Instead, turn your leaves over and check for white webbing and little spots. If you have a magnifying lens, look at the spots. You can also tap the leaf against a white sheet of paper and if any little dark spots appear, you probably have mites!
A way to help reduce the Spider Mite population is to spray the undersides of the leaves with a hard spray of water, much in the manner of ridding the leaves of Aphids. Keeping the dust in the area down (spraying the area with a mist of water) will also help. Use of pesticides/miticides is not recommended as 1) many become resistant and 2) these can endanger your butterflies! Again, pluck off the leaves and dispose of them. Leaves will become curled and can become spotted and discoloured from mite-damage.
So, if you do notice strange things, remove the leaves, throw them in the trash, wash your hands carefully before handling any butterfly eggs, larvae, or adults, and remember to only use healthy looking foliage when feeding your caterpillars!
Feeding your larvae 'bad' leaves may make them sick and who wants unhealthy caterpillars? Remember, poor food quality often = problems with completing a healthy life-cycle…