Like leaf rolling, leaf cupping is an easy-to-see warning that closer inspection is called for.
Cupped leaves tend to be thicker than healthy leaves. They may cup upwards or downwards. In some cases, they’ll do both. Sometimes, the conditions that cause cupping are temporary, and the leaves return to normal unaided. In other cases, the new shape is permanent. This can interfere with photosynthesis, and it creates safe havens for pests.
Leaf cupping can be caused by diseases, insect feeding, or physiological factors. Let’s take a closer look at each of those.
Diseases that cause leaf cupping
When fungal diseases take hold, they often block the flow of water and nutrients through the vascular bundle. Armillaria root rot causes leaves to cup downward. Eutypa dieback also causes cupping.
Several viral diseases can also exhibit leaf cupping:
Leaf cupping and insects
Aphid and thrips feeding often results in leaf cupping. Aphids tend to be more obvious than thrips, at least at first. Regularly checking on the underside of leaves can help nip these problems in the proverbial bud
Counter to popular myth, dish soap should never be used to eliminate these pests. Dish soap damages plants. Horticultural oils and castile soap are better options.
Physiological causes of leaf cupping
Extreme weather, nutrient deficiencies and toxicities, and chemical overspray can cause leaf cupping. Chemical overspray occurs when herbicides are applied when a breeze is present (or a car drives by). The chemicals end up in places you don’t want them, causing phytotoxicity.
Plants may cup their leaves on hot, windy days, especially when water is in short supply. They may also respond to heavy pruning and digging in this way. I suppose it’s something of a panic response. Most of us tend to withdraw when attacked, so I guess it’s no surprise that plants do something similar.
Deficiencies of boron, calcium, manganese, and molybdenum can result in cupping. Cupping may also indicate over-fertilization. Again, lab-based soil tests are invaluable when it comes to knowing what’s in your soil.
Plants may not speak in ways that we can hear, but we can certainly watch for things like leaf cupping as clues to what’s going on in the garden.
You can grow a surprising amount of food in your own yard. Ask me how!
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