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Copper containing compounds have a unique place in citrus rehabilitation due to their broad spectrum activity and longevity of their deposits
Edible Citrus, which originates from tropical and sub-tropical Asia, is perennial in growth habit. The most commonly cultivated species include Citrus sinesnis (sweet orange), C. reticulate (mandarin), C. limon (lemon) and C. aurantifolia (lime)—which comprise today’s commercial citrus in myriad of hybrid species and cultivars.
A well-managed citrus orchard has a long and fruitful life with trees invariably outlasting those who plant them. Initial investments are high and early rewards few during the initial growth stages as citrus trees normally start bearing fruit from the third year after planting, and economic yields are generally obtained from the fifth year onward. However, once the trees start bearing fruits, farmers can reap profit for at least the next 50 years, provided trees are well managed and citrus markets are receptive.
However, markets are fickle and small farmers in particular go through phases where they cannot afford the necessary management including pruning and disease control.
Citrus trees are prone to many different diseases which affect all parts of the tree including the root and collar region, trunk, branches, leaves and fruit. Another problem plaguing citrus especially in the humid wet tropics is epiphytic growth on the wood and foliage including algae, lichens, mosses and liverworts (Bryophytes), ferns (Pteridophytes) and bromeliads.
These problems require continuous management programmes and without proper and timely disease and epiphyte control citrus orchards rapidly fall into a state of disrepair. However, citrus is a hardy and resilient tree, and with care and attention can be rehabilitated and brought back into full production.
Citrus orchard rehabilitation
Diseases and epiphytes are suppressed by routine application of copper fungicides which are still the most cost-effective fungicides for citrus. However, even a short lapse in disease management allows trees to fall into disrepair and it requires rehabilitation for a return to full production.
Rehabilitation is the repair and rejuvenation of trees in a state of decline due to previous neglect. Pest and disease control plays a key part in rehabilitation. Having made all the initial investment, growers sometime allow trees to fall into a state of disrepair. Adding to the growers woes is the notoriously fickle agricultural commodity markets. Prices may be high at planting but ‘rock bottom’ five years later when the trees start to crop. It may make more economic sense for growers to minimise inputs, harvest what they can and switch future investment into profitable short- term cash crops.
But what comes down invariably goes up again and growers may suddenly be presented with opportunities from unexpectedly high prices, only to find their trees require remedial work within a programme of rehabilitation.
Structural rehabilitation of citrus including replanting any gaps and radical canopy management by pruning is the first and foremost task. This is followed by the suppression and control of diseases and the accumulation of epiphyte growth which is the decorative phase rehabilitation. That said, infection may be so severe that additional tree pruning may be required to excise diseased bark and wood, on top of that that required to restore tree shape and desired pattern of tree growth and cropping.
Copper as medicine and tonic
The copper ion (Cu2+) is best known as a fungicide but it is also an effective bactericide and molluscicide, as well as suppressing algae, lichens and even mosses, ferns and other plants (e.g. bromeliads), that grow on citrus trees in the hot wet tropics.
Copper has an important role in plant nutrition through operating in trace amounts as an essential micronutrient.
Evergreen citrus trees are prime and easy targets for foliar diseases because the leaves remain on the tree for such long periods of time. Citrus leaves can stay on the tree for up to three years although pest and disease load will reduce their longevity. Citrus foliage thus requires extended protection against fungal diseases, which can only be achieved by the most tenacious and rain-resistant deposits of protectant (protective or prophylactic) fungicide.
Copper fungicides are well-known for their ability to stick to leaves and fruit even in the face of the most intense tropical rainfall.
First copper fungicide was ‘Bordeaux’ a mixture of blue copper sulphate and strong alkali (lime), the latter used to overcome ultra-high solubility of copper sulphate and its potential for phytotoxic damage to plants.
This traditional copper fungicide was superseded by particulate fixed copper compounds such as copper oxychloride, cupric hydroxide, tribasic copper sulphate, copper ammonium complexes and red cuprous oxide. Cuprous oxide is generally considered to be the most active on a gram for gram basis as well as being the most tenacious.
Particle size distribution plays a crucial role in deposit tenacity and weathering resistance over and above any inherent high tenacity of cuprous oxide as a fixed copper fungicide. For instance, cuprous oxide manufactured by Nordox (Oslo, Norway) has all particles within the 1µm to 5µm diameter range (80 per cent less than 2µm and 99 per cent less than 5µm) to give an unrivalled particle-size profile.
The high surface area to mass ratio of very small particles maximises their adhesion to crop plant surfaces making removal by growth movements or weathering more difficult. Larger particles (3-4µm) are more likely to be blown off plant surfaces by air movements (wind), physically dislodged by plant growth movements or washed off by rainfall.
Rain simulation studies carried out in the Netherlands using a rainfall intensity of 10mm/hour recorded significantly superior deposit retention for cuprous oxide over cupric hydroxide and copper oxychloride. The median particle size of the cuprous oxide was 1µm which was significantly smaller than 3µm for the other fixed copper fungicides.
Foliar and fruit disease control
Research carried out on the island of Trinidad in the West Indies demonstrated how deposits of fixed copper fungicide were maintained at sufficiently high levels on the leaves of grapefruit trees to provide long term protection against Mycosphaerella citri (citrus greasy spot), Elsinöe fawcetti (citrus scab) and Diaporthe citri (citrus melanose). Rain simulation studies showed deposit resistance to the highly erosive effects of simulated tropical rainfall was the key factor maintaining longevity of deposits.
Many fungal pathogens will cause spotting, scuffing, scabbing and scarring of the citrus leaf surface but Gloeosporium limetticolum (wither-tip of lime) causes die-back of new shoots and leaves. Failure to synchronise sprays with appearance of new leaf flushes causes the tree to lose its leaf flushing pattern. Extended periods without spraying have led to misshapen lime tree canopies devoid of new leaf flushes and thus requiring resumption of cuprous oxide sprays synchronised with appearance of the new leaf flushes.
Citrus fruits are also affected by Phytophthora pathogens, more widely known for stem canker and gummosis of the trunk and branches of citrus trees but also infecting full low hanging fruit. Phytophthora spores are splashed up from the soil during heavy rainfall to infect low hanging fruit with a fast moving wet necrosis called brown rot.
This disease is avoided by spraying the fruit, and especially low hanging oranges, to run-off with cuprous oxide before the start of the rainy season.
An under-appreciated problem for the protection of citrus fruit is the dilution of surface fungicide deposit by fruit growth and enlargement. Research in New South Wales Australia showed how the surface area of lemons increased by a factor of 14 from the time of fruit set to harvest. This translates into a 14-fold dilution of fungicide deposit even without any loss from weathering. No protectant fungicide even highly tenacious cuprous oxide can withstand this sort of dilution through growth, so such ‘loss’ factors must be built into spray programmes through use of shorter spray intervals where necessary.
The broad spectrum activity of cuprous oxide provides for wide-ranging disease control. Fungal pathogens (other than those described above) susceptible to sprays of cuprous oxide include Colletotrichum gloeosporioides (anthracnose), Guignardia cirticarpa (black spot), Phytophthora parasitica (brown spot), Alternaria (brown spot and stem end rot) and Rhizoctonia (seedling blight).
Phytophthora canker and gummosis
Phytophthora may also infect the living bark of citrus trees to forms cankers which may bleed (gummosis). Farmers refocusing on trees after several years neglect will invariably find advanced bark infections at the collar or girdling the trunk (bole) and major branches all with potential to kill the tree. Cankers may arise in a range of positions on the tree, at the collar or just below soil level or on scaffold branches quite high in the tree, since infection may originate in the soil or from leaf and fruit infection higher in the tree.
Major branch infection is characterised by brown fluid oozing from the canker, earning the disease its other common name of ‘gummosis’. Gummosis is caused by physiological reaction of the tree tissues to the presence of the pathogen rather than being produced by the pathogen itself and is common problem on Pommelo trees producing the biggest of all citrus fruit which are very popular in South East Asia.
Canker infection can be cured by using cuprous oxide as a canker paint formulation which is brushed onto the affected part of the tree after cleaning the infected area. This entails cutting and cleaning away of all diseased bark and wood together with a sufficiently wide (several cm) buffer strip of healthy bark as insurance. The area is treated with a canker paint based on cuprous oxide together with adjuvant (e.g. sticker and surfactant) as appropriate. Farmers in South East Asia have achieved excellent results using Nordox cuprous oxide canker paint with addition of various oils and polymers to increase tenacity longevity and the penetration of the paint into the wood.
Epiphytes and molluscs
Algae and lichens growing on the leaves reduce the light trapping potential of chlorophyll in the leaf cells and thereby inhibit photosynthesis, growth and production. Larger epiphytes growing clearly add to the weight and load on the branches thereby increasing the risk of wind damage and breakage.
Epiphytic growth is suppressed during routine application of cuprous oxide for disease control and does not normally require extra dedicated spray applications. Cuprous oxide also kills molluscs (slugs and snails) which are soft-bodied animals with the ability and capacity to kill young seedling trees by stripping off all soft green foliage. Routine spraying of cuprous oxide in nurseries to keep seedling trees free from disease (e.g. Rhizoctonia) will kill these pests at the same time.
Dr Terry Mabbett