Treating Hollyhock Leaf Spot – Learn About Hollyhock Leaf Spot Control


By: Mary H. Dyer, Credentialed Garden Writer

Hollyhocks are charming, old-fashioned plants easily recognized by the tall spikes of colorful blooms. Although hollyhocks tend to be relatively problem free, they are sometimes plagued by leaf spot diseases, especially when conditions are warm and damp. Rust is the most common.

Recognizing Leaf Spot on Hollyhock

Hollyhocks with leaf spot display small spots which may be brown, gray, or tan, depending on the pathogen. As the spots enlarge, the dead tissue in the center may drop out, which give the leaves a “shot-hole” appearance.

The spots often run together to cover entire leaves when conditions are moist. In dry conditions, the leaves take on a speckled, tattered appearance. You may also notice tiny black spots which are fungal spores.

Hollyhock Leaf Spot Control

Hollyhock leaf spot diseases, which are usually fungal and less often bacterial, are spread primarily by wind, irrigation water, and rain. Leaf spot on hollyhocks usually isn’t deadly for the plant and chemical controls are rarely warranted; sanitation and proper irrigation generally keep the disease in check.

Water hollyhocks early in the day, using a soaker hose or drip irrigation system, or just let a hose trickle at the base of the plant. Avoid overhead sprinklers and keep the leaves as dry as possible.

Pick off affected leaves and twigs as soon as you notice them. Keep the area under and around the plants clean and free of dead and diseased plant matter. A thin layer of fine bark, pine needles, or other mulch will keep rainwater from splashing on the leaves. Limit mulch to 3 inches (7.6 cm.) if slugs are a problem.

Thin the plants if the hollyhocks are too crowded. Good air circulation can help prevent hollyhocks with leaf spot and even minimize the disease. Fungicides can be used when new growth emerges in spring if other methods of control aren’t effective. Read the label carefully to be sure the product is suitable for ornamentals.

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Damaged Japanese holly (Ilex crenata) roots infected with black root rot.
Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, www.forestryimages.org

Black Root Rot: Black root rot is caused by the fungus Thielaviopsis basicola. This fungus primarily affects the root system and reduces plant vigor. Aboveground symptoms may include stunting of terminal growth, shortening of internodes, and interveinal chlorosis. Infected roots are dark brown to black, usually starting at the root tips. Plants with extensive root rot damage will usually decline and die during dry periods.

Prevention & Treatment:The fungus has the ability to persist in the soil for many years, even in the absence of susceptible plants. High soil moisture and low soil temperatures favor the development of black root rot. Fungicide drenches are not generally recommended for landscape use since infected plants cannot be cured. Remove infected plants and replace them with other shrubs or resistant holly species, such as Chinese holly (Ilex cornuta). Yaupon holly (I. vomitoria) and American holly (I. opaca) are moderately resistant, while Japanese hollies (I. crenata) are very susceptible to black root rot. Use raised beds in landscape plantings to provide good drainage.

Phytophthora Root Rot: The water mold fungi, Phytophthora cinnamomic, and other Phytophthora species cause root rot on hollies growing in very poorly drained sites or wet areas. Planting too deeply and over-mulching may also contribute to disease development. The symptoms of this disease and black root rot are similar. Typically, yellowing of the leaves (particularly at the shoot tips), early leaf drop, slowed plant growth, and twig dieback are seen at early stages of the disease. Later, one or more limbs may wilt and die back to the main trunk, and a brown to black streak of dead tissue may extend from one area of rotted roots to the damaged limb. Often, the root system will continue to disintegrate until the plant dies.

Prevention & Treatment: Hollies grown under stress are much more sensitive to root rot disease than are well-maintained, vigorous plants. Always select hollies that are adapted to the local climate and soil conditions. Root rot pathogens are often introduced into the landscape on diseased container plants. To avoid introducing these pathogens, purchase hollies with healthy roots and good foliage color. Good cultural practices, such as proper fertilization, control of soil moisture, and providing good drainage (raised beds), will reduce the disease. Japanese hollies (I. crenata) are very intolerant of poorly drained soils and are especially prone to root rot.

The fungus thrives in areas with poor drainage and warm soils. Always choose locations that have good drainage for planting. The drainage of existing areas can be improved by using raised beds. Fungicides can be effective on a preventative basis only, and repeat applications are required. Fungicides containing mefenoxam can be applied in the home landscape but will not cure an infected plant. See Table 1 for examples of products containing the active ingredient. Due to product cost and for accurate application, homeowners may want to hire a licensed landscaper to apply products containing these fungicides. Apply all chemicals according to directions on the label.

Tar Spot: This disease is caused by the fungus Macroderma curtisii (formerly named Phacidium curtisii and Rhytisma curtisii). Yellow spots appear on the leaves of American and English hollies in May. These turn reddish-brown and finally black by fall. In years of heavy rainfall, berries, as well as leaves, are spotted.

Prevention & Treatment: Remove and destroy badly spotted leaves, prune to improve air circulation and overcrowding, and clean up and destroy fallen leaves.

Nematodes: Root-knot (Meloidogyne), ring (Criconemoides), stunt (Tylenchorhynchus), sting (Belonolaimus), and spiral (Helicotylenchus) nematodes are seldom seen due to their microscopic size. They live in organic matter in the soil or on roots and other parts of living plants. Most parasitic nematodes feed by a stylet, sucking juices from plant cells. They injure plants by direct feeding or wounding tissue, making an entrance for other disease organisms. Plant decline is often the only symptom, followed by gradual stunting, chlorosis, and leaf drop.

Prevention & Treatment: Presently, there are no effective chemicals registered for the control of nematodes in existing landscape plants. Remove infected plant material and surrounding soil. Plant resistant varieties into nematode-free soils. Chinese holly cultivar ‘Burford’ and Yaupon holly cultivar ‘Nana’ are tolerant to root-knot, stunt, and ring nematodes.


Hollyhocks

Although hollyhocks are considered biennials, they reseed themselves and return year after year, acting more as short-lived perennials. Hollyhocks produce large single or double blooms, 4 to 5 inches in diameter, in shades of red, pink, yellow, violet and white. The showy flowers sit atop huge spires, which can grow between 5 and 8 feet in height. Hollyhocks grow best in average, well-drained soil with a medium amount of moisture and full sunlight. In windy conditions or in rich soil, the plant's spires may require staking to support them. You can also plant hollyhocks along walls or fences to add a decorative touch and provide additional support for them.


Alcea rosea

Previously known as:

  • Althaea mexicana Kunze
  • Althaea rosea (L.) Cav.
  • Althaea sinensis Cav.

Alcea rosea, or Hollyhocks, are herbaceous flowering plants that reseed themselves and can produce colonies of plants that return in the garden year after year. They are typically found in cultivated areas and rarely in "the wild". Their original habitat is unknown, but the plant is probably a cultigen that started out in Turkey. Note that it is sometimes listed in nursery catalogs under Althaea.

The plant prefers full to partial sun, a heavy, rich, organic soil and seeds, sown in late summer or early fall, will flower the following summer. The plant tolerates a wide range of soil conditions and some light shade, but will not tolerate wet winter soils. Considered a biennial or short-lived perennial. It is a very ornamental plant and the flowers come in a number of various colors, from lavender to red to yellow, and resemble Papaver somniferum (poppies). The flowers grow on rigid, towering spikes of 5 to 8 feet tall and usually do not require staking. The plant has a long bloom period of June to August.

Diseases, Insect Pests, and Other Plant Problems:

Often grown as a biennial because it is susceptible to rust, leaf spot, and anthracnose. Slugs, spider mites, and Japanese Beetles can also be a problem. Lower leaves wither away during hot dry weather.

Juergen Mangelsdorf CC BY-NC-ND 2.0 Mauricio Mercadante CC BY-NC-SA 2.0 Jay Dot CC BY-NC-SA 2.0 Leaves Dinesh Valke CC-BY-SA 2.0

Control of Disease

There is no cure for a rust infection. At the first signs of rust, remove infected leaves and destroy them. Watch the plants through the growing season and remove any signs of the disease before it spreads. Since the fungus will overwinter, fall cleanup of plant debris is crucial for controlling the spread. Cut back and destroy all plant stems and collect all loose leaves from the plants. Also remove all mallow weed from the area. Do not compost infected plants. When new sprouts appear in spring, remove the first two leaves as an attempt to remove any spores remaining from the previous season.

  • There is no cure for a rust infection.
  • Watch the plants through the growing season and remove any signs of the disease before it spreads.

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