Hollyhocks are towering bloomers of the summer display. These old-fashioned favorites bring striking color and form to the back of the border amidst other perennial and annual flowers. A staple of cottage gardens, they attract pollinators in addition to their architectural beauty.
A curse among biennials is a fungus that targets plants specifically. Hollyhock rust affects members of the Malvaceae family. Once it’s established, it’s challenging to banish, but early detection and cultural preventatives help curb the spread.
What is Hollyhock Rust?
Small, rust-colored spots on leaf undersides are a common sign of this disease.
Hollyhock rust is the most common fungal disease among the species. The pathogen Puccinia malvacearum spreads to foliage, leaves, stems, and green flower parts. The fungus spreads quickly, and while it rarely kills plants, it causes discoloration, stunting, and premature leaf drop.
The disease affects lower leaves in spring and increases as summer progresses. It thrives in warm, humid conditions. Most leaves with rust dry or drop by the fall.
Mallows also host the fungus. It spreads via the invasive and widespread common mallow weeds Malva neglecta and rotundifolia. The weeds are reservoirs of the fungus, which travels quickly to nearby hollyhocks.
Identification
Rust disease causes unattractive, spore-filled pustules that discolor green leaves.
Hollyhock rust begins as yellow or dark red spots on leaves. Beginning with lower leaves, the infection makes its way up the plant. Spots are roughly ⅛ to ¼ inches across. Telltale orange-brown raised bumps appear on the undersides of leaves. They spread to leaf surfaces, stems, and sepals as the season wears on and infection becomes more severe. Irregular brown patches occur on stems.
The bumps aren’t pretty. They’re pustules that hold bundles of tiny fungal spores ready to erupt. Their rusty rounds discolor otherwise attractive, rich, green leaves. Leaves may get holes and become lacy, eventually withering and drying.
Causes
Overwintered spores infect new growth in warm, humid summers.
Puccinia malvacearum enters the garden on the wind or through infected plant material. The windborne fungus spreads locally through splashing rain and irrigation among plant parts and soils.
Spores thrive in warm and humid summertime conditions. They overwinter in hollyhock stems and leaf debris. Overwintered spores infest new growth as it emerges to repeat the seasonal life cycle.
Treatment
Once rust sets in, it’s challenging to eradicate. Early detection helps slow the spread. Non-chemical management is critical.
During the Growing Season
Clean pruning tools with 70% isopropyl alcohol after clipping hollyhocks.
Remove impacted leaves as soon as symptoms appear. Spotted leaves or the rust pustules themselves will show. If diseased sections are isolated, prune them off plants. Instead of adding the debris to the compost pile, dispose of them in garbage bags. Keep debris away from garden areas as spores continue to inoculate and spread.
After pruning or clipping hollyhocks, sterilize pruners to avoid spreading the spores between plants. As part of regular maintenance, sanitizing hand tools with rubbing alcohol (70% isopropyl alcohol) keeps tools clean.
While rust usually does not kill the plant, some growers use fungicides as a control or preventative measure. If you use fungicides, follow label instructions for proper application and alternate fungicidal active ingredients to best deter the disease.
Rust treatments include neem oil, sulfur, mancozeb, and other commercial fungicides that aren’t organically approved. However, as spores continually move in the wind, these may not be entirely effective.
End-of-Season Maintenance
Cutting and removing plant debris in the fall prevents overwintering spores.
As plants fade in the fall, cut stalks down to ground level. Thoroughly remove the stalks, leaves, and any debris from the site. Removal staves off spores that overwinter in dormant material. It protects future growth from encountering existing spores.
Prevention
Remove common mallow weeds to help reduce the spread of the fungus.
Cultural management is the best line of defense for the flowering biennials. Meeting their preferred growth requirements lays the foundation for sturdy performers.
But rust happens. To reduce its introduction, inspect nursery-grown plants before purchase. Don’t collect or grow new plants from seeds from previously infected plants.
Weed out and remove common mallow weeds growing in the vicinity. These widespread hosts readily transmit the fungus. Inspect nearby hibiscus, mallow, and rose of Sharon shrubs for signs of the disease. Weeding won’t account for your neighbor’s plants or the potential for spores to drift, but it may reduce localized spread.
Watering Methods
Drip irrigation or soaker hoses are best for root hydration.
Hollyhocks thrive in evenly moist, well-draining soils, especially as they establish. Mature plants are moderately drought-tolerant, though they grow and flower best with regular, deep watering sessions for consistent moisture.
Take care not to overwater the tall beauties, erring on the dry side. Rust spores and other fungi proliferate in damp conditions, and prolonged periods of moisture provide an ideal habitat.
Watering at the base of plants, rather than overhead, is best to deliver moisture to roots while avoiding splashing the leaves and potentially harboring fungus. Drip irrigation or soaker hoses allow measured delivery at low volumes with little runoff.
If you rely on overhead sprinklers to irrigate, run them in the morning. This gives plants time to dry in the day’s sun and breeze.
Plant Spacing
Space plants at least 18 inches apart to reduce the risk of fungal growth.
Allow plenty of air circulation between plants to reduce humid conditions. Crowding hollyhocks in a cottage garden or border display is tempting, but they benefit from breathing room.
Ample airflow between plants or structures lessens the propensity for fungus. Depending on the variety, space plants 18 inches or more apart.
Fertilizing
High nitrogen levels spur leafy growth but can weaken plants.
Hollyhocks adapt to various soil types but grow best in soils high in organic matter. In rich soils, they don’t require additional nutrition.
Amending with organic matter in poor or lacking soils helps produce vigorous plants. If hollyhocks lack blooms or show yellowing leaves and moisture and sunlight aren’t the issue, there may be a nutrient deficiency. Fertilizing your plants with a balanced fertilizer or one with a higher phosphorus (K) level may help with blooming.
Avoid overfertilizing. High levels of nitrogen spur leafy growth but may cause overall plant weakness. Overfertilizing creates plant stress and is the perfect storm for fungal rust.
Plant Selection
Alcea rugosa features tall, lemon-yellow blooms above gray-green foliage.
Disease-resistant hollyhock varieties may successfully ward off some strains of the infection. The ‘Halo’ series in apricot, blush, cerise, and lavender varieties boasts improved disease resistance and gorgeous blooms. Plants reach heights of five to six feet tall.
The fig-leaf species Alcea ficifolia resembles classic hollyhocks in form and flowering without concerns about rust. The deeply lobed foliage resembles fig leaves in shape. In bright cultivars like ‘Happy Lights’ and ‘Las Vegas,’ stems grow six to eight feet tall.
Russian hollyhock, Alcea rugosa, bears lemony yellow blooms atop resistance plants. Stems reach six to seven feet tall above gray-green basal foliage. This unique species is a reliable and showy performer among the genus.
When growing hollyhocks, interplant with perennials and annuals to disguise declining leaves. The towering blooms will shine while rust-ridden bases hide behind other dynamic flowers and foliage.
Frequently Asked Questions
Hollyhock rust seldom kills the host plant. It causes discolored leaves, stunted growth, and declining foliage.
The rust is an airborne fungus whose spores spread locally via wind, splashing rain and irrigation, and infected plants.
This specialized fungus affects plants in the Malvaceae family, including hibiscus, rose of Sharon, abutilon, and lavatera. Common mallow weeds also host the fungus. It easily transfers among the family.
