I work at a vineyard where we have hundreds of Hollyhocks growing. A lot of them have yellow/orange wart like spots on the leaves aggregating outward. The leaves eventually turn yellow and drop off the plants. We sprayed them last week with a copper sulfate/sulphur mixture but it doesn’t seem to have done any good.
Can you please tell me what is wrong with the plants?
Is it a pest, fungal disease or mineral deficiency?
This sounds like a case of Hollyhock Rust, Puccinia malvacearum. This is a fungal infection that is most prevalent in wet or humid weather, such as we endured this past spring & early summer. The fungus is wind borne but needs moisture on plant leaves to infect and spread amongst plants. Hollyhocks, Alcea Rosea, are susceptible because they easily self seed and grow very close together.
Below, I have outlined a few remedies that should help break the infection cycle, control the outbreak and help with future management:
* Now, dispose of any diseased plants, leaves, stems, & roots. * Continuously, clean up any leaf or other debris from the soil around the plants. * Be vigilant in watching for further spread and immediately take action, as previously stated. * Thin out plantings in order to lower humidity, allow more air circulation through the plants and let leaves dry off faster. * Avoid over-watering or under-watering to encourage deep root growth. Use of drip irrigation is preferable rather than overhead sprinklers as leaves do not get wet. * Cut plantings down to soil level in the fall and dispose of plant material – do not put in compost. * Any tools used should be sterilized/disinfected. * Buy new Hollyhock seeds for next year – do not collect seeds from these plants. * If plants are started from seed, start them far away from any old plants or that site. * Any new purchased plants should be thoroughly inspected for disease before being put in the ground and then monitored carefully.
* Avoid planting Mallow species, such as Abutilon, Hibiscus, Lavatera, Malvastrum, Malva, Sidalcea or Malva sylvestris, anywhere nearby.
* If weather remains wet and infection continues to spread, a fungicidal spray may be needed. This should be done 2 times per week when the plants are in rapid growth. A garden maintenance professional who is licenced to spray should be consulted.