Here are some of the pathogens and pests that affect plants in the Malvaceae family in California:
Pathogens
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Hollyhock Rust (Puccinia malvacearum). Plants can easily get P. malvacearum from metal fences. Orange-brown pustules on the underside of leaves are indicative of this disease.
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Powdery mildew (dusty white powder covering leaves and stems)
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Botrytis blight has a characteristic gray fungal growth
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Leaf spots caused by pathogens such as Alternaria, Cercospora, Colletotrichum, or Septoria spp., create sharply defined necrotic lesions on mallow leaves.
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Antrhacnose is a disease that causes dark, sunken lesions on the leaves and stems.
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Verticillium is a soil-borne fungus that is difficult to get rid of. Wilting leaves are a symptom.
Pests
- The mallow flea beetle prefers mallows and hollyhocks. This beetle is usually up to 5 mm (0.2 inches) long and can be identified by its yellow-red head and dark blue wing coverlets. Mostly, these pests are not dangerous to the plant.
- The Mallow Leaf Miner (Calycomyza malvae) causes damage to the leaves.
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Bactericera lavaterae (a psyllid that causes galls on Island Mallow)
- Other pest problems include Japanese beetles, spider mites, snails and slugs.
References:
On May 26, 2024, I noticed several Malva nicaeensis plants of very different sizes. The smallest one was no bigger than the palm of my hand (in breadth and height). The largest one was waist high (approx. 35 inches). Very interesting! Especially in light of the fact that Jepson eFlora states that M. nicaeensis only grows to 2-6 dm (23.6 in) tall.
Sample 1: https://www.inaturalist.org/observations/218510391
This plant was very tiny - no bigger than the palm of my hand (~3 inches in diameter; ~1.4 in tall). The flowers and fruits were also very tiny. This plant was growing in the hard-packed dirt on the trail under full sunlight.
Sample 2: https://www.inaturalist.org/observations/218510400
This plant was as tall as my hand (~6 in) and only a few inches wide. It was growing in the shade under sycamore and California bay trees.
Sample 3: https://www.inaturalist.org/observations/218510444
This plant was twice as wide as Sample 1, but the flowers and fruits were about the same size. It was also growing in hard-packed dirt in the middle of the trail in full sunlight.
Sample 4: https://www.inaturalist.org/observations/218640472
This one was knee high (~20 inches). It was growing in the shade under some oak trees. The tips of this plant had been nibbled on - perhaps by the nearby herd of cows, so it was probably once taller than the measured height.
Sample 5: https://www.inaturalist.org/observations/218640481
This one was lying prostrate on the ground, but when I raised the stem, it was waist high (~35 inches). There were three main stems and lots at the leaf axils along the stem. It was growing with milk thistles along the side of the trail in full sun.
Sample 6: https://www.inaturalist.org/observations/218640479
I also noticed some M. nicaeensis leaves infected with some sort of virus. This is a new observation for me.
