blk spots on tomatoes

(Question)

please look at the pictures and tell me what the black dots are that on on my yellow grape tomatoes…I origianlly thought spider poop but some of the tomatoes are on the outside of the plant

(Answer)

Thank you for contacting Toronto Master Gardeners and sending a clear photo of the affected fruit of your yellow grape tomatoes.

Tomatoes can be susceptible to several problems during a growing season. Some of these problems are infectious diseases which affect the leaves, fruit and roots of the plant and are relatively easy to identify. Often these problems result in reduced yield and affect the quality of the fruit. Other problems “relate to growth conditions associated with soils, cultural practices and weather” and can cause dis-figuration of the fruit–blossom-end rot, fruit cracks, cat-facing and uneven ripening, which are also easy to identify.  Since you mention nothing about the actual plants showing signs of disease, it’s likely that only the fruit  has been affected. Your tomatoes don’t seem to have the obvious problems, so by the process of elimination, one might consider a virus of some sort.

Cornell University has an excellent web-site that shows fruit symptoms of tomato disorders. The only one on that site which resembles the picture of your grape tomatoes is alfalfa virus. See: <http://vegetablemdonline.ppath.cornell.edu/DiagnosticKeys/TomFrt/TomFrtKey.html#Alfalfa>. However, the virus is transmitted by aphids between alfalfa plants and tomatoes. If there is no alfalfa growing near you, it is doubtful that alfalfa virus is the problem.

Finally, another possibility is bacterial speck which is caused by bacteria which survive in the soil and plant debris and can be dispersed by rain. Since the Toronto area has had cooler and wetter weather this summer, it would be reasonable to suspect that this is the cause of the spots on your tomatoes.  See: <http://utahpests.usu.edu/IPM/files/uploads/PPTDocs/13sh-diseases-veg.pdf>. [This site also shows the alfalfa virus affected tomatoes].

Neither of these disorders will affect the eating quality of the tomatoes.  However, there are a few things to keep in mind to control bacterial speck in the future.

  •  At the end of the season, discard or destroy any affected plants.
  • Do not compost them.
  • Rotate your tomato plants yearly to prevent re-infection next year.
  • Do not share seeds from affected plants.
  • Make sure to use proper spacing when planting and water plants from below, as bacterial speck on tomatoes spreads quickly from plant to plant in crowded, cool, wet conditions.

Hope that this has helped to identify the ‘black dots’ on your tomatoes and that you will enjoy eating them. And, of course, there is always next summer.