Mallow Pests and Diseases: Hollyhock Rust, Sawfly and Flea Beetle

Hollyhock rust is the main event

Hollyhock rust attacks the whole mallow family and mallow is fully susceptible. It shows first as small yellow or orange spots on the upper surface of the lowest leaves, with raised orange-brown pustules directly beneath them. Left alone the pustules multiply, the leaf yellows and dies, and the infection moves steadily up the plant until the stems are affected and the plant is bare below the flowers.

The disease does not blow in from nowhere. It comes from hollyhocks, from other ornamental mallows, and above all from common mallow growing as a weed in rough ground, verges and hedge bottoms nearby. If there is a rusty hollyhock within fifty feet (15 m) of your plants, you will get rust, and controlling it is a matter of slowing it rather than preventing it.

It also overwinters. Spores survive on fallen leaves, on old stems left standing, and on the crown of a perennial plant. A patch that was rusty last year starts rusty this year unless you clear it properly at the end of the season.

1. Check the undersides weekly from late spring Turn over the lowest three or four leaves on each plant once a week. The pustules appear underneath first and are obvious once you know to look. Catching the first ten spots is worth more than any treatment applied later.
2. Remove infected leaves the moment you see them Pick off every leaf with pustules, put it straight in a bag, and bin it. Do not compost it and do not drop it on the soil. Removing a dozen leaves in early summer can hold the disease at the base of the plant for weeks.
3. Strip and clear the bottom of the plant Take all foliage off the bottom six to eight inches (15 to 20 cm) of stem and keep fallen leaves raked out of the crown. Rust needs still, humid air and splash from below; both are removed by opening the base.
4. Deal with the source, not just your plant Pull mallow weeds within reach of the bed and, if you can, cut down badly rusted hollyhocks nearby. Treating your own plants while a rusty source stands twenty feet away is wasted effort.

Cut every stem to the ground in autumn and remove all debris, including fallen leaves and mulch that has leaves in it. Leaving old growth standing over winter is the single most reliable way to guarantee rust again next season.

Judging when a plant is beyond saving

There is a point at which stripping leaves is just prolonging a plant that will not flower well. Use the flowering stems as the test rather than the leaf count: while the upper third of the plant is clean and producing flowers, keep it. Once pustules reach the flower stems themselves, the plant is a spore factory sitting next to your healthy ones.

Removing one badly infected plant early usually saves the rest of the row. It is a hard call in midsummer and an easy one in hindsight.

Mallow sawfly and flea beetle

Two chewing pests show up reliably. Sawfly larvae are small, pale green, caterpillar-like grubs that feed in groups on the underside of the leaves and reduce them to a lace of veins within days. They work from the middle of the plant outward and are easy to miss until the damage is obvious.

Flea beetles are tiny, dark and jumpy, and they attack seedlings rather than mature plants, peppering the young leaves with small round holes. A mature mallow shrugs them off entirely; a two-inch seedling can be killed outright.

Hand picking works on mallow because the plant is open and the pests feed in obvious groups. Ten minutes twice a week through early summer handles sawfly completely without any spray.

Problems that are not pests at all

Several common mallow complaints are growing conditions rather than organisms. Reading them correctly saves you spraying for something that is not there.

The pattern to look for is whether the symptom follows the plant's structure. Damage that is worst on the oldest, lowest leaves is usually disease or age; damage spread evenly over the whole plant is usually water, light or nutrition.

Keep a rust diary

Rust is a repeating annual problem, which makes it exactly the kind of thing worth recording. The date you first see pustules is remarkably consistent on the same site, and knowing it lets you start stripping leaves a fortnight before it appears rather than a fortnight after.

Record positions as well as dates. Rust reliably starts in the same corner of a bed, usually the one with the least air movement, and after two seasons of notes that pattern is undeniable.