Horticulturist, gardening consultant, and designer Jonathan Garner suggests strategies for tackling a nasty fungus spreading throughout Australia’s south-east corner.
Who uses Indian Hawthorn?
The numerous cultivars are low-maintenance evergreens with glossy foliage and showy spring flowers. They tolerate heat, frost, drought (once established) and coastal conditions. Their bulletproof suitability for most garden styles and applications has put them in most specifiers’ plant list – up until now, that is.
Entomosporium – or Photinia – Leaf Spot is among the most common and destructive diseases affecting Raphiolepis. The disease becomes apparent in winter or spring when small red spots appear on the leaves and quickly multiply. If left unchecked, the disease can cause the shrub to lose most of its foliage. The continual loss of foliage gradually weakens the plant and its appearance.
Although it rarely kills the plant, once it takes hold in the garden, it’s pretty much there to stay.
Survivor
The fungal pathogen thrives in cool, moist and humid conditions. It’s encouraged by periods of frequent rain or overhead watering, and the cooler wet weather in autumn really gives it a run on. It mainly attacks plants in the rose family, and Raphiolepis is one of its most susceptible hosts. Other plants affected include Photinia and Pear cultivars.
The fungus survives through winter on infected leaves and fallen debris. When conditions are wet, spores are released and splash onto new leaves. Young foliage is especially vulnerable, which is why infections are often worse in spring as plant’s new leaves emerge.
Signs
What to look for:
• Tiny circular red or purplish spots on young leaves;
• spots, usually 1–2 mm across, which get larger and develop a grey or tan centre;
• in heavy infections, the spots may merge, forming larger irregular patches;
• older leaves turning orange or reddish before dropping;
• significant leaf drop occuring during winter and spring;
• plants appearing thin and bare, with leaves mainly at the branch tips; and
• shrubs losing their compact, dense appearance.
Although the disease seldom kills the plant outright, repeated loss of foliage weakens it. Poor health makes the shrub more vulnerable to heat stress, cold damage, pests, and other diseases. Given Raphiolepis has become a core member of popular plants in the landscape asset, if this disease is left unchecked, we’ll all be in strife.
I’m seeing it throughout the NSW South Coast, Southern Highlands, Central Tablelands, Blue Mountains, and it’s now creeping into areas of Sydney. My colleagues in Victoria are noticing the same spread.
What encourages Entomosporium?
Understanding what the fungus likes helps to reduce risk. Entomosporium spot thrives when:
• Leaves stay wet for long periods (heavy dew, rain, overhead irrigation);
• humidity is high;
• temperatures are cool to mild;
• new growth is present; and
• plants are crowded with poor air movement.
This is why coastal and cooler regions often see more severe outbreaks, while dry inland areas may see fewer problems.
Management
Naturally prevention is better than a cure.
The most effective way to manage the spot is through effective cultural practices. Here are some helpful strategies:
• Use resistant cultivars. Some newer Raphiolepis varieties have been bred for better disease resistance (and some haven’t). Unfortunately, we have yet to compile a list of resistant cultivars in Australia. Frustratingly, the lists from overseas generally don’t include our cultivars. Replacing highly susceptible plants can be easier than constant
treatment. I’ve found Oriental Pearl, Cosmic Pink, Hot Tips, and X Intermedia are prone, whereas Apple Blossom, Snow Maiden, Cosmic White and Umbellata species are more resilient.
• Improve air flow. The pathogen loves still, damp air. Although it’s often easier said than done, try to space plants apart, prune interior branches to open the shrub, and avoid planting in shady or crowded beds.
• Reduce leaf wetness. Avoid overhead sprinklers. Water at soil level or use drip irrigation and water early in the morning so leaves dry quickly.
• Ensure garden hygiene. Remove and dispose of fallen leaves and infected pruning material. Don’t compost this debris, as spores survive and spread from infected litter, and it’s best to rake the infected leaves as blowing out beds will naturally spread the spores.
• Store the diseased material in a bulk bag. What do you think will happen with the uncontained spores when you drive down the road? If you have leaf spot in one of your gardens be prepared to have it in others. It’s contagious.
• Maintain healthy plants. A strong plant is better able to withstand infection. Fertilise lightly in early spring with higher levels of potassium and calcium to toughen the foliage and avoid fertiliser with excessive nitrogen that encourages faster and weaker growth.
Once the spot has taken hold you’ll need to implement an annual chemical regime to effectively manage its recurrence. If you’ve got it this year, then, without adequate prevention, you’ll have it next year.
Fungicides can only help protect the new growth. They don’t cure leaves already infected. Sprays work best as part of an
integrated management plan that includes sanitation and improved cultural conditions. Beyond lifting your cultural-practices game and using chemicals, you will definitely need a calendar to remind you to get on the front foot next autumn. Fungicides work best as preventatives. Spraying once the spots are present will, at best, only help to keep it at bay. If you’re onsite fortnightly or monthly without a preventative regime, you and the plants are in trouble.
Chemicals
Effective chemicals at preventing the spot include:
• Tombstone Duo (Trifloxystrobin and Tebuconazole), labelled ‘Caution’;
• Systhane (Myclobutanil), labelled ‘Caution’; and
• Procon 250EC (Propiconazole), labelled ‘Poison’.
All of these chemicals are banned for use on edible plants. They are potent fungicides that must be respected. It’s your responsibility to apply them as the label instructs and to wear appropriate PPE.
All of these chemicals are systemic, which means if the healthy foliage is thoroughly covered while the plant is actively growing, the chemical will be absorbed and provide a few weeks of protection. Continued use of the same product will likely result in the pathogen becoming resistant to that specific chemical. Remember our last issue when I touched on modes of action? This is important with fungicides. Fungus can develop resistance quickly. It’s best to apply the same product twice then change to a product that’s listed in another group (aka ‘mode of action’).
There’s nothing effective that’s sold at Bunnings.
Some colleagues have found regular spraying of a phosphite such as Agrifos or Yates Anti Rot helps a plant’s resistance. Although phosphites aren’t labelled for effective control, they certainly can’t hurt.
For best results:
• Read and thoroughly obey the instructions;
• begin spraying in late autumn before the spots begin to show in winter;
• spray at intervals listed in the instructions; and
• stop in late spring when the weather warms and new growth has hardened.
If fungicides alone are relied upon while watering practices remain the same and fallen leaves are allowed to accumulate beneath, results will be disappointing. Sprays help most when combined with cultural changes.
Pruning and removal decisions
For light to moderate infections, prune to improve air flow, then implement watering and sanitation changes, and spray the developing foliage as soon as possible.
For severe infections with repeated defoliation, pruning helps, but may not solve the problem if the plant is genetically susceptible.
For chronic problems year after year, cut your losses and replace the shrub with a more resistant variety or an entirely different species. This often provides the most long-term success.
So, sadly, Nature has found a chink in the armour of this once bulletproof shrub. Following my instructions listed here should see things improve, but be ready for Entomosporium again next year.