November 9, 2004
From: ProMED-mail<promed@promedmail.org>
Source: British Society of Plant Pathology, New Disease Reports, Vol. 9 [edited]

New report of _Alternaria alternata_ causing leaf blight of tomato in Pakistan
K.P. Akhtar, M.Y. Saleem, M. Asghar and M.A. Haq, Nuclear Institute for Agriculture and Biology, Jhang Road, P.O. Box 128, Faisalabad, Pakistan. Accepted for publication: 30 Jul 2004.

During February 2004 a leaf blight disease of tomato (_Lycopersicon esculentum_) was observed in a tomato field at the Nuclear Institute for Agriculture and Biology, Faisalabad, Pakistan.

Symptoms on affected plants started with yellowing and browning of the lower leaves, progressing upwards under high humidity conditions. Symptoms often developed from the leaf tips and along the margins of the leaf petiole.

Under severe infection, lesions enlarged and coalesced causing blighting of the leaves. Concentric circles with dark layers of spores were observed under moist conditions on blighted leaf portions. Infection under favorable
conditions was found to cause severe defoliation, with considerable yield losses when it occurred prior to flowering.

Isolations were made on PDA from infected leaves collected from the field. The fungus _Alternaria alternata_ [Aa] was consistently isolated and identified based on morphological characters (Ellis, 1971; Sultana, 1981; Shakir et al., 1997). The fungus grew well on PDA and formed greyish-black colonies of about 90 mm in diameter in 7 days, when incubated at 25±2 deg C.

Conidia formed in long chains and were obclavate and muriform, often with a short conical or cylindrical, pale beak, less than one third of the length of the conidium. Conidia had 3-7 transverse septa and usually several
longitudinal or oblique septa.

Pathogenicity tests were carried out in a screen house on 8-week-old tomato plants of the cultivar 'Pakit'. Test plants were sprayed with a conidial suspension (50 000 conidia per ml) of Aa isolated from diseased tomato
leaves. Control plants were sprayed with sterilised distilled water. All test plants were covered with polyethylene bags for 2 days. Leaf blight symptoms similar to the ones observed in the field started to develop after 4 days and Aa was consistently reisolated from these plants. Control plants did not develop leaf blight symptoms.

Aa has previously been reported in Pakistan as a saprophytic pathogen of tomato causing post harvest losses in high frequency (Akhtar et al., 1994). Among 35 Aa isolates collected from rotted fruits from fields and markets
only 1 isolate from the field was able to produce leaf blight symptoms. This shows that Aa causing leaf blight in the present study is a distinct pathotype.

This is believed to be the first report of Aa causing leaf blight in tomato plants in Pakistan.

References

Akhtar KP, Matin M, Mirza JH, Shakir AS, Rafique M. 1994. Some studies on the post harvest diseases of tomato fruits and their chemical control. Pakistan Journal of Phytopathology 6, 125-129.

Ellis MB, 1971. Dematiaceous Hyphomycetes. Kew, UK: Commonwealth Mycological Institute.

Shakir AS, Mirza JH, Akhtar KP, 1997. New records of Alternaria species from Pakistan. Pakistan Journal of Phytopathology 9, 102-104.

Sultana K, 1981. Cultural studies of Hyphomycetes of Pakistan from different habits. Punjab, Pakistan: University of Punjab, MSc thesis.

ProMED-mail
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[Aa is the most frequently reported _Alternaria_ species causing plant disease. It is primarily a weak pathogen, attacking already stressed plants, but may also be pathogenic on healthy plants. It is interesting to note that all of the previous posts related to Aa concern citrus. Wounding is required, especially during harvest, leading to post-harvest and storage infections of tubers and fruits, or by insects and nematodes. Insects facilitate infection and act to stress the entire host-plant system, thereby reducing overall plant resistance. Furthermore, insects may act as vectors by dispersing inoculum throughout the crop. Infection pathways include nutrient, water, or microclimate-originated stresses. Recommendations for disease management include a 3-year crop rotation for
tomato or potato, planting of crops that are not hosts of Aa, removal and burning of infected debris and eradication of weed hosts to further reduce inoculum. Combining resistant cultivars and other control measures greatly decreases disease progress.

Additional links: <http://gis.ucsc.edu/disease/Fungal%20Pathogens/Alternaria/Alternariaportrait.html>
<http://vric.ucdavis.edu/veginfo/commodity/tomato/tomblackmold.pdf>
- Mod.DH]