How To Remove Black Spots From Citrus Plants/Leaves
Black spot disease is a problem for many citrus growers which can make plants and fruits unsightly and therefore unsellable. African Pegmatite is the supplier of copper oxychloride Demildex as an effective treatment and prevention measure against black spot disease in citrus and a whole host of other plants.
Citrus plants are a popular choice for commercial growers as they provide for fruits highly sought after by consumers. Generally, given good weather and a healthy crop, production will be strong year after year. However, problems arise when crops are threatened by diseases or pests. For growers that have invested time and energy into citrus trees, it can be quite frustrating losing part of the harvest due to these threats, in addition to being financially punishing. This article will take a look at the causes of these diseases and how they can be treated .
Citrus black spot disease in most cases is not fatal, but often causes early fruit drop and a reduced overall yield. Black spot disease does, however, reduce the visual appearance of fruit which can impact their selling ability. It is not unheard of that a significant price reduction can be levied on even the most mildly impacted fruits. In 2003, the European Commission temporarily banned the import of citrus fruits from South America due to the risk of transmission of black spot disease to the European crop.
What is a Citrus Plant?
Biologically, citrus is any tree or shrub that belongs to the Rutaceae family. Originally found in most parts of Asia and the northern parts of Australia. Through adaptations over centuries, various species of the plant have been adapted to suit the environment where they are grown. There are quite a number of citrus trees all over the world now. The plant genus is very adaptable and can flourish almost anywhere, as long as the conditions are right, preferably not too cold. Popular citrus plants include; Oranges, Limes, Grapefruits, Tangerines, Lemons and many more.
What Causes Dark Spots on Citrus Trees/Plants?
As earlier said, problems arise when citrus trees are threatened by diseases or pests. A common cause for alarm is noticing black sooty spots on citrus plants, especially their leaves or the fruit itself. These dark spots are a result of fungal infection. The fungus; Guignardia Citricarpa, appears in the form of a black mold that is chalky, gritty to touch on leaves and is responsible for dark spots appearing on the fruits. The mold is the result of insects feeding on the plant. Some of these insects are aphids, mealy bugs, citronella scale, and whiteflies. As the insects feed, they leave behind honeydew, which facilitates the easy spread of the fungus. The mold, when it spreads, covers the surface of the leaf and hinders photosynthesis. Hindering photosynthesis can stunt the growth of the tree without outrightly killing the tree.
As with all kinds of disease, the infection causes a reduction in productivity. Reduced productivity translates to significant economic losses. Lemons and Valencia orange plants are more susceptible than other citruses to black spot disease caused by G. Citricarpa. There are no known citrus varieties that are immune to the disease.
In addition to via insects as above, transmission of the fungus via ascospores forming overwinter in fallen leaves, with these ascospores being ejected from the leaf pile by rain and/or watering whereupon they are moved to new plants by the wind and rain. Upon reaching a new plant, the ascospores germinate as appresoria which leads to the infection.
How can the infection be prevented?
As the saying goes, prevention is better than cure. In addition to standard citrus growing best practices, the following may be useful for aiding in the prevention of black spot disease in citrus:
- Pruning the trees in spring and pollinating them in early summer or even earlier in late spring. It will help to prevent the trees from growing leaves in the fall, a period that insects are most active.
- Another option is to water the trees. Keeping the trees well-watered helps prevent insect infestation. Insects like aphids are more active in dry periods and areas of drought. So keeping the plants well-watered will likely prevent infestation. A drop in infestation level is very likely to result in a corresponding reduction of honeydew production. If the honeydew is not produced as much, the likelihood of widespread fungal infection is drastically reduced.
- Treating the tree after harvest is good practice too. The tree can be treated with some special chemicals or oils after the fruits have been harvested to help it maintain its healthy state. One such recommendation would be Imazalil oil. It is advisable to be careful when handling such oils, to avoid causing harm to humans in general.
- Acceleration of the decomposition and/or destruction of the end of season fallen leaf pile should be prioritised, especially before heavy rains, so as to limit the potential spread of ascospores.
How can the infection be treated?
Thankfully treating the dark spot is not as hard as treating other diseases. It is crucial to start treatment as soon as possible. Infected trees can be treated as follows:
Determine the infected plant/tree
A quick check will reveal plants that already have the dark spots on their leaves. The dark spots are a definite mark of infection. Also using a lens to check for insects is a prudent measure. The insects tend to live on the stem, branch crotches or the undersides of the leaf. Checking the plant branches for ants is good practice. Insects go around the tree looking to collect honeydew. Therefore, the presence of ants or other insects is a good indication of infection.
Determine the level of infection
After checking the plants and noting the ones with the infection, it is equally important to determine the level of infection. It could be a small infection, which is preferable. It means the infection was caught at the onset. In this case, spraying the plant with a strong blast of water will do the job of getting rid of the insects. The water will also get rid of what little honeydew is present. Also pruning the infected foliage can help prevent the spread of the fungus.
Begin treatment
In the case of large infections, other steps have to be taken. Application of horticultural oil is the best bet. But before applying the oil on a large scale, it is best to dab a little on the leaves of plants. This should be done at least a couple of days before applying the oil. It will help growers identify the trees/plants that are sensitive to the oil. After taking note of the sensitive trees, cover them up before spraying the oil. The spraying can be done again after a 10 to 14 day period to help get rid of severe infestations.
The oil can be paraffin or neem-based if the prevalent insects are aphids or whiteflies. Spraying should be done early in the morning or late at night. In general, spraying is best done at periods of low humidity. The temperature should be between 4 to 32 °C. Spraying should be done until the leaf surface is completely drenched in oil before moving on to the next tree/plant, ensuring that the undersides are sprayed too. The oil does its job of suffocating the harmful insects while remaining harmless to beneficial insects and causes no damage to the plant itself. Furthermore, the oil application makes it harder for ascospores to adhere, reducing the efficiency of that transmission method.
The mold leftover can then be removed after the insects are dead or gone. A water spray can get the job done in most cases, but a damp cloth soaked in a soap and water solution can also be used for accessible branches.
For some of the most pernicious cases, treatment using chemical means may be necessary. Copper oxychloride Demildex, from African Pegmatite, is a highly effective chemical treatment for black spot disease in citrus plants.
Treatment with copper oxychloride Demildex
What is copper oxychloride?
Copper oxychloride Demildex is a highly tailored antifungal treatment for a variety of fungal diseases affecting flowering and fruiting plants. The specifically tailored formulation of copper oxychloride for agriculture provides optimal quantities of copper to effectively treat and eliminate fungal attacks. Copper compounds have long been used as antibacterial and antifungal treatment agents for a broad range of concerns. With a tightly regulated production process, copper oxychloride Demildex delivers the essential copper without any other harmful heavy metals present, which can have detrimental impacts on plant growth. Demildex is a light green powder containing 850 g kg-1 of copper oxychloride as the sole active ingredient.
How can Demildex be used to treat the problem?
Standard approaches for the treatment of black spot disease in citrus plants would use highly soluble copper oxychloride Demildex in a 2,500 ppm (0.25 vol%) solution in water in the early stages of the growing season, using misting sprayers to ensure good coverage over the leaves and new shoots. Good solubility is ensured by Demildex’s particle size of 95% below 5 μm and a surface area in excess of 30,000 cm2 g-1.
Demildex is applied in water and is further dissolved on the leaves when there is some kind of weak acid present. For the transmission and evolution of black spot in citrus, Demildex has more than one mode of activity.
Copper, and thus Demildex, is toxic to insects; therefore reducing any honeydew produced. Furthermore, it can become dissolved in the honeydew. Through both the honeydew vector and by direct contact with the fungal spores, Demildex therefore inhibits their transmission by interfering with the reproductive process.
Notes on the use of copper-based chemical fungicides
As with all copper based soil and plant treatments, care needs to be taken when spraying with copper oxychloride Demildex. Excess application can lead to elevated levels of copper in the soil, which can cause lower productivity levels in the plants and can leach off and potentially cause a eutrophication event. At least 14 days should pass between the last spraying and harvesting. Special care should be taken to avoid ingestion by humans or animals on account of copper’s toxicity in even mildly elevated levels.
Summary
- Black spot in citrus plants is a fungal disease caused by Guignardia Citricarpa
- Left untreated, it can cause unsellable crops, early fruit drop and yield reduction
- Adherence to citrus growing best practices is helpful in preventing the spread of the disease
- Chemical treatment with copper oxychloride Demildex is highly effective at eradicating the disease
African Pegmatite is the supplier of copper oxychloride Demildex, a highly effective treatment for black spot disease in citrus plants, providing fruit growers the confidence and ability to guarantee a sustainable and productive crop, year after year.
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