Help! The fruit on my lemon are distorted and covered in ugly warts. What’s gone wrong? Can my tree be saved? Is there an organic remedy? Answer: An infectious fungal disease called lemon scab is responsible. And yes, this infection can be defeated organically and in more than one way. Read on…
Lemon scab (Elsinoë fawcettii or E. australis) disease infects Rangpur lime, lemonade, mandarin and lemons. Bush lemons are especially susceptible to damage. This fungus occurs wherever these citrus are grown in climates with a warm, wet, humid climate. In Australia, lemon scab occurs all along the east coast.
Affected fruit become distorted and covered in lumpy warts and scab-like growth making them look revolting. Citrus scab also attacks the leaves, affected foliage developing slightly raised, irregular scabby outgrowths. Initially, the scabs are grey or pinkish then they darken with age, but they are more common on fruit than leaves. Infected fruit are often the first thing a gardener notices.
The fruit can be juiced, but they’re useless for marmalade. Apart from being unsightly, if this disease isn’t controlled it will end the productive life of the tree and it is very likely to infect other susceptible citrus.
There is ‘no rigid framework’ for the remedy, so you need to be proactive depending on the growth patterns of your plant in your garden.
Copper hydroxide is an approved organic input and by spraying to completely cover the affected plant you begin the recovery to health. Complete coverage means spraying all the flowers, twigs, bark and both sides of leaves so the entire plant has a protective layer against this fungus. So make sure you are relaxed and do not rush the job. Make three applications three weeks apart.
Infection occurs through soft, new leaves and flowers, so the ‘three spray response’ begins with every flush of new growth or every flowering, starting with whichever comes first.
There are measures which further reduce the risk of infection:
* Spray at dawn before honeybees are active;
* Cease foliar feeding citrus and never wet the foliage by hand watering or by using sprinklers. Water sitting on leaves enables the fungal spores to germinate, penetrate and infect citrus. Persistent dew can also help infection.
* Rainfall can wash the protective layer of copper off citrus, which is why three timed applications is important.
* Prune infected trees, removing all dead, damaged or spindly growth. If there are crossing or rubbing branches, remove them. The aim of pruning should be to encourage an increase in air flow, which helps accelerate the evaporation of dewfall.
* In my experience at Sydney Botanic Gardens (1992-2003) adding two drops of sunflower oil and one drop of dishwashing detergent to every litre of spraying mix can result in even coverage and help the fungicide to adhere to foliage.
Wear gloves and a long-sleeved shirt when applying copper hydroxide.
Note that all copper-based fungicides enable copper to accumulate in soil, and copper is know to gradually affect beneficial mycorrhizal fungi. Copper hydroxide is an approved organic input because it is slightly less harmful than copper oxychloride (the latter being the most widely used copper fungicide).
Ultimately, successful control using fungicide depends on the type of citrus, how badly infected it is, and how diligently the spraying is done. Expect control to take around year, maybe more.
Environmental health and healthy orchards
Some growers prefer to cull infected trees rather than attempt the lengthy and costly process of disease control in an orchard. It’s quicker than spraying, it reduces the risk of spreading an infectious disease, and it avoids introducing copper into the soil. Apart from being the most immediate solution, it is also the most environmentally responsible. Over to you…
Director, Seed Savers’ Network
Patron, National Toxics Network