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I have two hoya plants of this variety - one very old and one only about a year old. They both have red/brown dots on the underside of some of the leaves. On the older plant the brown spots show up intermittently on the plant, not just around the soil line. On the younger, most leaves are close to the soil and most leaves are affected. My guess from reading the previous posts is the problem is fungal. If so should i remove the affected leaves (probably not) and would Soap-Shield - a copper fungicide - or liquid sulfur be okay to treat them with?
They were both outside plants last summer in Minnesota. I never noticed the problem until returning home after 2 months of having a plant sitter care for them. Different watering regimen? Thank you for any suggestions.
If you think its fungal an old knowing hoya person told me to spray with athletes foot spray make sure its aerosole.if its not fungal it wont hurt. if you have affected leaves by the soil I would remove them. check the soil make sure its not staying to wet.maybe this helps alittle.
There's a simple, safe and inexpensive cationic surfactant (detergent) that is an effective fungicide, bactericide and viricide that is routinely used in orchid culture that I've been also using successfully with Hoyas. It's the same stuff that's used as an antiseptic for cuts that goes by the name benzalkonium chloride. The trade name of the most available concentrate goes by the name "Physan 20", which is a 20% solution. It can be easily purchased at some nurseries and many orchid growers. A good dilution to use is one teaspoon per gallon. It works best for surface infections.
A pint should cost less than $10 and would last a lifetime for most gardeners.The stuff is clear and very stable such that a dilute solution can sit for many years without losing potency.