Bacterial leaf spot of hardy geranium: prevention and control

This page provides details of the prevention and control of bacterial leaf spot (caused by Xanthomonas hortorum pv. pelargonii of hardy geraniums.

This information was last updated in 2023.

Bacterial diseases are very difficult to control once disease has become established in a crop and symptoms are widespread. The best means of control is prevention through the use of clean (pathogen-free) propagation/planting material and good nursery hygiene. It is important to remember that the pathogen may be present in the absence of disease symptoms.

Clean start

Stock plants

We have shown that the production of disease-free hardy geraniums is perfectly feasible when careful attention is given to the health status of the parental material.

Stock plants or mother-plants used to provide propagation material should be kept separate from production material. Ideally they should be grown under protection and watered via a sub-irrigation system. They should be tested for the presence of Xhp, and any plants testing positive or showing symptoms should be discarded. When propagating, wash and disinfect hands and secateurs/knives regularly, ideally between stock plants.

Plug plants

Many production nurseries obtain plug plants from specialist propagation nurseries, this means they have little control over the health status of the propagating material. Due to the high intensity of production, there is much greater potential for rapid spread both within and between batches of plants from different origins. Disease symptoms may also not be apparent at the time of despatch to the production nursery. Freedom from symptoms does not mean that young plants are free from the pathogen. It is therefore important to check with suppliers that material has been tested/indexed to an appropriate standard. Based on a spread experiment done as part of Project CP 191, a health standard of <0.03% (infested plants) would need to be achieved in liners to ensure disease incidence did not exceed 10% for production outdoors.

Irrespective of the above, all bought-in plant material should be quarantined and carefully checked over several weeks for the presence of symptoms; the entire batch should be rejected if any symptoms are found.

Production

Once potted, grow on batches from different sources as far apart as possible; this will minimise the risk of cross-contamination between batches.

Cultural practices

Bacterial pathogens are easily spread by water splash, it is therefore important to minimise overhead irrigation and use sub-, drip- or trickle-irrigation systems wherever possible. Similarly, increasing the spacing between individual plants will also reduce the rate of spread.

The rate of plant-to-plant spread will be greatest in plug plants with overhead irrigation. A system for growing plug plants using a sub-irrigation system has been shown to be feasible, for more informations  see research project CP 191.

Soft, lush growth leads to increased susceptibility to infection. It is therefore important to avoid over nutrition, and another reason to keep watering to a minimum, i.e. grow plants as hard as possible.

Nursery hygiene

Bacteria can be passively spread by anything that comes into contact with an infected plant or contaminated surface. It is therefore important to develop a culture of good hygiene amongst all staff across the nursery.

Staff

Staff should wash/disinfect hands when moving between crops/batches. One way to encourage this is to issue all staff with their own disinfectant hand gel, and/or place dispensers at entrances/exits to glasshouses. It is particularly important to try to avoid the movement of machinery and staff within and between susceptible batches of plants when the plants are wet.

Cuttings, pruning and trimming

Taking cuttings, pruning and trimming are potentially highly effective ways of spreading bacterial diseases; not only are the bacteria spread on cutting blades, but they are simultaneously inoculated into a fresh wound. Outdoors, such operations should only be done during periods of dry weather, not just dry on the day of the operation, but forecast to be dry for the next few days until wounds have become more resistant to infection. Pruning and cutting tools/knives/secateurs should be disinfected as often as possible during operations, and certainly when moving between different batches.

Clean up

It is good practice, for any disease, to remove and dispose of diseased or contaminated plants, leaves and prunings as quickly as possible.

Bacterial pathogens can survive for long periods in dry crop debris, or in dried-on slime or ooze from infected plants. It is therefore important to remove crop debris and thoroughly clean down and disinfect surfaces and structures between crops/batches and particularly after infected crops.

Disinfectants

The efficacy of disinfectants is dependent on having sufficient contact time. Most standard disinfectants are likely to be effective against bacterial plant pathogens when used at recommended concentrations in long soaks.

When working at a bench it is better to have at least two implements in use, so that one can be left to soak in disinfectant solution for as long as possible. Out on the nursery, this is not practical, but the results from research project HNS 179 suggest that the use of iso-propanol impregnated disinfectant wipes are a good compromise.

Pesticide sprays

Attempts at control with pesticide sprays are unlikely to be cost-effective, as significant spread has most likely already occurred when symptoms are seen.

Currently (2022), there are a limited number of approved biological products (Serenade ASO, Amylo-X WG, Botector) that may have activity against bacterial plant pathogens, but previous work has shown the reductions achieved may not justify the costs.

Previously, HDC trials have shown that frequent sprays with copper oxychloride (Cuprokylt) can give significant reductions in the rate of spread of bacterial diseases. However, the use of copper sprays is no longer approved.