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Plantwise Technical Factsheet

mango scab (Elsinoë mangiferae)

Host plants / species affected
Mangifera indica (mango)
List of symptoms/signs
Fruit  -  abnormal shape
Fruit  -  lesions: scab or pitting
Fruit  -  premature drop
Growing point  -  dieback
Growing point  -  lesions
Inflorescence  -  lesions; flecking; streaks (not Poaceae)
Leaves  -  abnormal colours
Leaves  -  abnormal forms
Leaves  -  abnormal leaf fall
Leaves  -  necrotic areas
Symptoms
The symptoms of mango scab are extremely diverse depending on factors such as the plant part affected, cultivar, age of tissue at infection, inoculum potential, water and mineral nutrition (expressed as plant vigour and lushness) and possibly the amount and distribution of free water. Only young tissue is susceptible to infection, for instance fruit is no longer susceptible after it reaches about half size. The occurrence of all symptoms is dependent on the availability of free water when the tissue is at the susceptible stage. Some of the symptoms can be confused with physical or insect injury or infection with other diseases (Condé et al., 1997a).

Fruit

In Darwin, Australia, the most noticeable symptom is on the fruit (Condé et al. 1997a). Initially small black lesions form on the newly set fruit. These lesions can be easily confused with the black lesions of anthracnose on newly set fruit and heavily affected fruits fall off the tree. Lesions on the fruit of the cultivar Kensington Pride, which remain on the tree, develop into light-brown scabs or scar tissue, either as small scabs or as large, irregular scar tissue when the lesions coalesce. As scabs develop they consist of scar tissue with a central scab which can, in some cases, be lifted off. Anthracnose infection does not produce this type of scar tissue on the fruit. If there are only a few fruits affected the disease can be confused with abrasion injury. More diverse lesions occur on the cultivar Irwin, which is popular in Darwin, Australia. These lesions range from small black spots, which could be mistaken for spray injury, to small and large scarred areas, the large areas being accompanied with a depressed distortion of the fruit. The scarred areas in all cultivars could be mistaken for damage caused by insect injury. However, with mango scab, there is no indication of any chewing to the fruit and significant numbers of potentially damaging insects will not be found. Of the two cultivars investigated in some detail in the Darwin area, Irwin has been found to incur greater damage than Kensington Pride. Unlike anthracnose, mango scab lesions do not develop into a soft rot as the fruit matures.

A scab or scar tissue is formed after Amblypelta bug injury and this is associated with a disorder called 'lenticel blowout' (Smith et al., 1997). In contrast to mango scab, injury by Amblypelta causes a much deeper depression in the fruit skin. Lenticel blowout tends to occur on fruits when they are half grown; it is more evenly distributed on the fruit and tends not to be prominent on mature fruits.

Stem, inflorescence and fruit stalk

The most common symptom on stem tissue is the occurrence of numerous slightly raised, grey, oval to elliptical lesions. If conditions are somewhat dry, the lesions will be smaller and black. Lesions on the inflorescence or frutescence may initially appear similar to those of anthracnose, however, on closer inspection or microscopic examination they are seen to be raised structures in contrast to the non-raised lesions of anthracnose. Another symptom consisting of large, light-tan, corky areas, resembling the scar tissue caused by insect injury, has been observed on stems. Diverse lesions occur on the cultivar Irwin, which is popular in Darwin, Australia. These lesions range from small black spots, which can be mistaken for spray injury, to small and large scarred areas, the large areas with a depression of the fruit. The scarred areas in all cultivars can be mistaken for damage caused by insect injury.

Leaves

A wide range of symptoms has been observed on the leaves although these symptoms are largely overshadowed by the more dramatic damage on the fruits. Common symptoms are: brown necrotic spots with halos; edge lesions associated with hydathodes; corky lesions on the lower leaf surfaces; or elongated, dark lesions along main veins under the leaf. However, elongated lesions along main veins can also be caused by Amblypelta spp. and infection by algal leaf spot, Cephaleuros virescens. In wet weather, numerous small, brown, necrotic lesions or shot holes may form on young leaves leading to defoliation. Other symptoms on leaves are lesions with central scabs and numerous small lesions about 0.1 mm diameter along secondary veins. The former can easily be dismissed as not being mango scab while the latter can be confused with mechanical abrasion. Leaves often appear distorted due to the effects of marginal or edge lesions and other lesions on the growth and expansion of the leaf.

Nursery

In nurseries a similar range of symptoms (shot hole, numerous small necrotic lesions, distorted leaves) occurs on the leaves as occurs in orchards but these tend to be more prominent on the young growth. Defoliation is common in severe infections. Small, black or elongated, grey scab lesions are also found on young stem tissue.
Prevention and control
Cultural Control

In severe scab infections on trees or nursery stock, it may be beneficial to prune away old infected stems to reduce the levels of inoculum.

Chemical Control

Copper fungicides (oxychloride, hydroxide or oxide) need to be applied from at least flower bud emergence to flowering, and then after the fruit has set till the fruit are half-grown, in order to protect the fruit from infection. Copper fungicides mixed with certain other chemicals can cause phytotoxic burning symptoms on mango tissue (Condé et al., 1997a). It is, therefore, advisable to replace the copper sprays with mancozeb during flowering and fruit set. Experiments in Darwin, Australia, indicate that the use of copper fungicides alone will not cause damage to flowering or fruit set. Where copper sprays are used against flowering anthracnose, mango scab may be undetectable.
Impact
If controlled, mango scab should cause little economic damage. Without chemical control, losses as high as 90% have been observed in one orchard during an investigation in 1996-97 in Darwin, Australia (BD Condé, NT Department of Primary Industry and Fisheries, Darwin, Australia, unpublished data).

Related treatment support
Plantwise Factsheets for Farmers
Kenya, Kengap Horticulture Ltd; CABI, 2012, English language
 
 
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