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Crops
Copper protection for the ‘Eye of the Dragon’ Longan (Dimocarpus longan) is one of a number of tropical and sub-tropical trees bearing sweet, succulent fruits. All are members of the Sapindaceae or soapberry family OAPBERRIES INCLUDE MANILKARA zapota (sapodilla), Nephelium lappaceum (rambutan) and Litchi chinensis (litchi or lychee). The last is the closest to the longan in botany and morphology and therefore the most easily confused. Litchi fruit (2.5 to 3.8cm in diameter) are borne in loose terminal clusters and have a yellow/pink/red rough and leathery skin. Pulp or aril is white, firm and translucent. Longan fruits are slightly smaller (2.2 to 3.6cm in diameter) and borne in tight clusters of rounded fruit with smooth and pliable leathery skins enclosing a similarly white and translucent aril. At maturity fruit skin colour ranges from tan to light brown. By then, the skin is brittle and bark-like and, therefore, easy to crack and peel off from the aril. Dimocarpus longan is a medium-sized, evergreen tree reaching six to seven metres in height. The dark green leaves are covered with thick wax cuticle to give the upper leaf surface a distinct gloss or shine. Longan trees are intolerant of frost, prefer free draining sandy soils, dislike poorly drained soils and are intolerant of water-logged soils which can kill the trees within four to five days. However, unlike litchi they will tolerate exposed and windy sites but not saline soils near the coast. They require a cool and dry frost-free winter for good flower set followed by a warm summer with moist soil conditions to ensure maximum fruit production. Longan is now grown in sub-tropical areas of North America such as Florida and in Queensland, Australia, but is still very much an East Asian fruit, widely and intensively grown in the northern areas of Thailand and Vietnam, in Laos, Cambodia, and Taiwan, but most at home within its native distribution across southern China. The Chinese call the longan ‘Eye of the Dragon’ because of its appearance; the jet black shiny seed seen through the translucent aril closely resembles an eyeball.
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The tan-coloured fruits are borne in large tight clusters (Photo: Omex)
Diseases and their control Disease spectrum of longan and litchi is somewhat similar. Longan is not troubled by a single major disease but is attacked by a wide range of wood, leaf, flower and fruit infecting pathogens which collectively cause significant economic losses for this exotic and high value sub-tropical Asian fruit. Longan is susceptible to infection by a wide range of true fungal pathogens and fungus-like pathogens (e.g. Phytophthora). This makes the broad spectrum activity of copper containing fungicides, especially cuprous oxide, the most active of the fixed copper fungicides, the most appropriate choice.
Diseases in the tree nursery Foot rot caused by the Fusarium fungal pathogens is widely reported from Florida and Queensland, Australia, and southern China where nursery losses from this disease regularly reach 10 per cent. Pythium species and Rhizoctonia solani which are the classic damping off and foot rot pathogens of a wide range of tree seedlings have been reported in litchi. The use of broad-spectrum action copper fungicides such as cuprous oxide and applied as a drench is an established and proven choice for controlling the equally wide range of pathogens which cause these damping off and foot rot diseases of nursery trees.
Foliar diseases Phomopsis leaf blight caused by Phomopsis guiyuan and Phomopsis longanae is widely reported in southern China along with a similar looking leaf blight caused by Cladosporium oxysporum. Infected and dead leaves which are the principal sources of fungal inoculum
should be removed. Systemic fungicides like benomyl and thiophanate methyl are recommended. However, in addition to a high risk of fungicide resistance development there is the ever present threat of algal spot through not using copper containing fungicides. For this reason growers are advised to ‘play safe’ and use the broad spectrum protectant action of cuprous oxide which will also control algal spot.
Phytophthora on leaves and fruit Foliar blight and fruit rot caused by Phytophthora palmivora can be a significant problem for longans grown in northern Thailand, especially where fruit set occurs in the ‘off season’ when cool weather follows on from two to three days of rainfall. Trees in northern Vietnam are also at risk of infection. Young shoots, panicles and fruit are the worst affected parts of the tree. Range of symptoms includes a dark necrosis on young shoots, brown coloured blight of the inflorescences, distinctive brown lesions on the fruit and premature drop of both flowers and fruit. Systemic actives such as metalaxyl from the acyl alanine group of fungicides were recommended and used. However, with the onset of widespread resistance in Phytophthora pathogen populations to this mode of fungicide action growers of longan and many other tree crops have resorted to the use of cuprous oxide and other broad spectrum action protectant copper-based fungicides.
Algal spots Algal spot (Cephaleuros virescens) is a problem wherever longan is grown and especially so in wet climates. Copper
FAR EASTERN AGRICULTURE Issue Two 2016 | www.fareasternagriculture.com