Cladosporium species and Alternaria alternata cause serious post-harvest early calyx decay (PHECD) in truss tomatoes in Israel

A. Bustan¹*, S. Cohen², O. Erlich³, and L. Tsror (Lahkim)³

¹ Ben-Gurion University of the Negev, The Blaustein Institutes for Desert Research, Bergmann Campus, P.O. Box 653, Beer-Sheva 84105, Israel
² Arava Research and Development, Sapir Center, D.N. Arava 86825, Israel
³Agricultural Research Organization, Gilat Research Center, D.N. Negev 85280, Israel

*abustan@bgu.ac.il

Accepted for publication 30/07/07

Truss tomatoes imply freshness and excellent taste; consumers are highly attracted to the tomato aroma provided by the green parts of the cluster and enjoy the ability to pick tomatoes from the cluster over several days. However, in the past two years, a new syndrome, referred to as post-harvest early calyx decay (PHECD), has become a major quality problem in the trade of truss tomatoes. Although graded at harvest and packed as high-quality fruit, 5-30% of the fruit clusters that reach European markets 7-10 days later are sold at low prices, repacked, or simply dumped due to shriveled calyces (Fig. 1, 2), or being frequently covered with black mold (Fig. 3) or active mycelia (Fig. 4).

Figure 1: Typical dehydration of cherry tomato sepal leaves following five days of storage at 12°C and relative humidity of 90%. Light microscopy showed active hyphae of Cladosporium and A. alternata	Figure 2: Shriveled calyces of truss tomatoes stored for five days at 25°C and high relative humidity. Light microscopy showed active hyphae of Cladosporium

Cladosporium species and Alternaria alternata identified by microscopic observation (Barnett & Hunter, 1998) were the prevalent fungi (92%). These pathogens were recovered following surface disinfection of the calyces, indicating that they had colonized the truss as latent disease already in the greenhouse. During storage (12°C), the mycelia expand gradually from the sepal tips to the calyces and then to the peduncles and, finally, to the rachises, all of which eventually shrivel or dehydrate while uninfected trusses remain fresh and green (Fig. 5). The severity of the PHECD syndrome increased during the warm seasons and declined in the winter. Cladosporium isolates from overtly infected calyces were identified as C. sphaerospermum (Penzig) and C. tenuissimum (Cooke) by the CBS Identification Service, The Netherlands.

Figure 3: Typical black mold, identified as conidia of Cladosporium, on the sepal tips of cherry tomatoes. In extreme cases, these can already be observed in the greenhouse.	Figure 4: Active hyphae of A. alternata on shriveling calyces of truss tomatoes following 5 days of storage at 25°C and high relative humidity.
Figure 5: Most truss tomato products still reach the markets with the desired fresh, green calyces and rachis; uninfected cherry tomato following five days of storage at 25°C and high relative humidity.

The latent nature of the disease at harvest does not facilitate confident selection of uninfected clusters on which to conduct pathogenicity tests with the aim of completing Koch's postulates.

Previously, C. tenuissimum was associated twice with dry rot of tomato fruit (Fajola, 1979; Narain & Rout, 1981) and Stacey (1977) mentioned C. sphaerospermum to cause sooty mould on the honeydew of whitefly during glasshouse production of tomatoes. However, this is the first time that these two Cladosporium species and A. alternata, a well-known post-harvest pathogen, are reported to promote the shriveling of tomato calyces and rachises.

Acknowledgements

We thank AGREXCO Ltd. for supporting this study.

References 

Barnett HL, Hunter BB, 1998. Illustrated Genera of Imperfect Fungi, 4^th Edition. Minnesota, USA: The American Phytopathological Society Press. 

Fajola AO, 1979. The post-harvest fruit rots of tomato (Lycopersicum esculentum) in Nigeria. Nahrung 23, 105–109.

Narain A, Rout GB, 1981. A tomato rot caused by Cladosporium tenuissimum. Indian Phytopathology 34, 237–238. 

Stacey DL, 1977. 'Banker' plant production of Encarsia formosa Gahan and its use in the control of glasshouse whitefly on tomatoes. Plant Pathology 26, 63-66.