Vegetarian, 05-05 - May 2005

Water and Pathogen Infiltration into Tomato Fruit
P. R. Gilreath, Manatee County Extension Service
J. E. Bartz, Plant Pathology Department
J. W. Scott, Gulf Coast REC - Balm
S. A. Sargent, Horticultural Sciences Department

Fruit decays such as soft rot are continual problems in tomato packinghouses in Florida. Microorganisms, both plant and human pathogens, enter produce in a number of different ways. Fruit damage such as punctures, wounds, cuts or cracks are common entry routes and these injuries can occur both in the field and in the packinghouse (Fig. 1). Damage can be caused by wind-blown sand and other mechanical injuries. Pathogens such as soft rot are ubiquitous and normal inhabitants of vegetable fields; they multiply during moist weather. Fruits picked when the plant is wet will have a film of water covering the stem scar and pickers can spread pathogens to fruit surfaces in the field during harvesting. Insects, birds and even dust can serve as vectors for both plant and animal pathogens, especially after fruit injury. Once in fruit tissue, through either the stem scar or wounds, bacteria cannot by killed by chlorine or any other sanitizer/disinfectant.

Water infiltration in dump tanks can be a major source of contamination; a considerable amount of research has been done to better determine where the problems are and to develop management strategies to minimize them. Research has shown that tomatoes with fresh stem scars are more vulnerable to infiltration than tomatoes with dry stem scars, and green fruit were more likely to take up water than pink or red fruit (Fig. 2). Fruit immersed to deeper depths or in water less than 10 F warmer than the fruit pulp temperature will also take up more water. Since the porosity of the stem-end scar increases with fruit pulp temperature, harvest during hotter months presents greater potential problems. Some varieties also have large air spaces inside the fruit. These air spaces will constrict when submerged in cooler water and as air space volume decreases, water will be drawn into the fruit, especially at the stem-scar end. This problem is addressed when standard packinghouse procedures are followed.

Water congested stem scars are particularly vulnerable to an invasion by bacteria. Water congestion occurs when fruit are infiltrated by water. Additionally, there's some evidence that stem scars are congested with water when wet fruit are pulled from the plant.

Water infiltration rates may also vary by tomato cultivar. Work conducted by Drs. Jay Scott and Jerry Bartz at the University of Florida demonstrated significant differences in water infiltration among several common tomato varieties. Table 1 illustrates water uptake by fruit in a study conducted in Bradenton in 2004. FL 47 and FL 91, two commonly grown cultivars, had the highest amount of water uptake, compared to Sebring which was the lowest. Table 2 shows results from uptake work in 2003 in Bradenton and Quincy.

Because there are a number of variables which may affect the infiltration of water and pathogens into tomato fruit, both field and packinghouse managers must be aware of potential problem areas in order to minimize problems. For packinghouse handling guidelines, see 'Handling Florida Vegetables - Tomatoes' by Steve Sargent, available online at http://edis.ifas.ufl.edu/VH079. Also, UF/IFAS publication HS866, 'Identifying and Controlling Postharvest Tomato Diseases in Florida,' has been updated and expanded and is available at http://edis.ifas.ufl.edu/HS131 .

Figure 1.  Fingernail wound in fruit that developed into black mold rot.

Figure 2.  Infiltration of blue dye through stem scar into internal tissue.  

Both photos credit: M. J. Mahovic