First Record of Sooty Spot Caused by Cladosporium cladosporioides on Mango (Mangifera indica L.) in Egypt

Mango (Mangifera indica L.) is an economically important fruit tree belonging to family Anacardiaceae, grown in the tropical and sub-tropical areas of the world. It is usually grown in the nursery and marketed as a scion seedling. Mango fruits are consumed due to their high nutritional values, delicious taste, and excellent flavour. In January 2022, abundant fungal infection appeared in scattered areas on the leaves of mango seedlings at a commercial seedlings production nursery in Ismailia Governorate, which is considered one of the most important districts for mango production.
The development of symptoms occurred when seedlings were covered during low winter temperatures with a plastic sheet (polyethylene), which leads to more internal moisture around the seedlings. More than 70% of seedlings in the nursery showed symptoms of the disease. Symptoms appeared in the form of small, irregular shaped necrotic black spots on the upper surface of the leaf, while on the lower surface of the leaf corresponding black spot, appeared as a branched network of mycelium, the color of which ranged from gray to dark gray (Fig. 1A and B).
To determine the etiology of this pathological phenomenon, diseased leaves were used for isolation by using potato dextrose agar (PDA) medium and incubated at 25°C for 5 days. The colonies showed mycelium, olive-green to brown, with a velvety texture (Fig.2A). The fungus colonies were isolated from all samples and on the basis of their cultural and morphological characteristics, the isolated fungus was identified as Cladosporium cladosporioides (Fresenius) de Vries, according to the descriptions of (Tashiro, et al., 2013). The pure culture of the fungus was prepared by the single spore technique.
The representative isolate Cladosporium cladosporioides EGYARC1 conidiophores are solitary, straight to slightly curved, olivaceous-brown, narrowly cylindrical to subcylindrical-oblong. Ramoconidia are oblong to cylindrical, with an average length and width of (14.73 µm [10.97 to 24.47 µm] × 3.48 µm [3.25 to 4.60 µm]). Conidia are catenated, in long branched chains that were readily disarticulate, single celled, and subspheric to limoniform, olivaceous to olive-brown in color with visible scars and denticle-like extensions projections were quite typical of the Cladosporium genus. The mean diameters of length and width of conidia are (5.95 μm [4.58 to 7.27 μm] × 3.7 μm [2.82 to 4.60 μm]) (Fig.2B, C, D and E).
To confirm the identification, the ITS region was amplified by PCR and sequenced using the primer set ITS1/ ITS4, by following the method described by White et al., (1990). The sequence was compared with the existing sequences in GenBank, and the result BLAST searches showed 99.79 to 100% identity with the sequence of Cladosporium cladosporioides (Fresenius) de Vries (GenBank Accession No. KT959335.1 and KT959303.1), which confirmed the morphological identification. The ITS sequence was deposited in GenBank (ITS, OP854844).
A pathogenicity assay was carried out to confirm Koch's postulates. Five seedlings of healthy 1-year-old M. indica (var. Sukkary), originated from seeds, were inoculated with a 5 mm-mycelial plug of C. cladosporioides from a 5-day-old culture grown on potato dextrose agar medium. Inoculation was done on the surfaces of young leaves. The leaves of five control seedlings were inoculated with sterile PDA plugs. All seedlings were covered with plastic bags and removed two days after inoculation. All seedlings were kept under greenhouse conditions at 25°C under natural daylight conditions. 15 days after inoculation, the inoculated plants showed typical symptoms similar to those previously observed in the nursery, while the control plants remained healthy. The experiment was performed twice. Koch’s postulates were fulfilled when typical colonies of C. cladosporioides were successfully re-isolated from the leaves showing the same symptoms.
To the best of our knowledge, this is the first report of C. cladosporioides causing sooty spot disease on mango (Mangifera indica) seedlings in Egypt.