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Bacterial spots, specks and septoria leaf spot

Published Jan 30, 2023

Bacterial spots, specks and septoria leaf spot

The three infections are very similar and can at times be hard to distinguish them. We will look at some of their similarities, and differences, as well as the control measures.

Bacterial speck 

It is caused by the bacterium Pseudomonas syringae pv tomato.   Leaf symptoms of bacterial speck consist of small black spots (approximately ⅛ to ¼ inch in diameter) that often are more prominent on the undersides of leaves.  As the spots age, a yellow halo may develop around the edge.  Spots on fruit are very small (almost pinpoint-like) and do not penetrate very deeply into the tissue.  The spots can be raised, flat or sunken, and range in color from brown to black.  On unripe, green fruits, the spots often have darker green haloes, while on ripe fruits the spots can have subtle, yellow haloes

Bacterial-speck-tomato.png

Bacterial speck in tomato fruit

Bacterial spot 

It is caused by the bacterium Xanthomonas campestris pv. vesicatoria. Bacterial spot develops on seedlings and mature plants. On seedlings, infections may cause severe defoliation. On older plants, infections appear as water-soaked areas. Leaf spots turn from yellow or light green to black or dark brown. Older spots are black, slightly raised, superficial, and measure up to 0.3 inch (7.5 mm) in diameter. Larger leaf blotches may also occur, especially on the margins of leaves. Symptoms on immature fruit are at first slightly sunken and surrounded by a water-soaked halo, which soon disappears. Fruit spots enlarge, turn brown, and become scabby.

Bacterial spot in tomatoes.jpg

Bacterial spot in tomato fruit

Septoria leaf spot or Septoria blight 

It is caused by the fungus Septoria lycopersici. This fungus can attack tomatoes at any stage of development, but symptoms usually first appear on the older, lower leaves and stem when plants are setting fruit. Symptoms usually appear on leaves but can occur on petioles, stems, and the calyx. The first symptoms appear as small, water-soaked, circular spots 1/16 to 1/8" in diameter on the undersides of older leaves. The centers of these spots then turn gray to tan and have a dark-brown margin. The spots are distinctively circular and are often quite numerous. As the spots age, they sometimes enlarge and often coalesce. A diagnostic feature of this disease is the presence of many dark-brown, pimple-like structures that are readily visible in the tan centers of the spots. When spots are numerous, affected leaves turn yellow and eventually shrivel up, brown, and drop off. Defoliation usually starts on the oldest leaves and can quickly spread progressively up the plant towards the new growth. Significant losses can result from early leaf-drop and often leads to the subsequent sun scalding of the fruit when plants are prematurely defoliated. 

Septoria leaf spot in tomatoes.jpg

Septoria leaf spot in leaves

Some similarities and differences across the three diseases:

  • Both bacterial spots & specks symptoms appear on the leaves, stems and fruits. Septoria blight symptoms appear on the leaves and stems but do not extend to the fruits
  • Yellowing of leaves is more pronounced in bacterial spot and septoria blight
  • Bacterial spot & specs attack mostly on the leaf margins and tips
  • Septoria leaf spot initial symptoms appear on the older lower leaves and gradually extend upwards. In most cases, this is not the case with bacterial spots & specks as symptoms may also appear first in younger leaves
  • On fruits, bacterial speck lesions are slightly raised but are generally much smaller (1/16 in) than those of bacterial spot. Bacterial speck lesions are very superficial and do not crack or become scaly as in bacterial spot.   
  • On immature fruits, the spots in bacterial speck may be slightly raised while in in bacterial spot, the initial spots are mostly sunken. 
  • Bacterial spot also infests peppers

Management of bacterial spots and specks:

The two diseases have similar control measures. Once they have already set in, there is currently no known cure. The best is to try and manage the diseases' aggressiveness to just salvage some harvests. Here are a few control measures that can be undertaken;

  • Avoid/minimize working on the crops when the leaves are wet as this may only spread the disease to unaffected plants or parts. The spores can be spread through hands and clothing of pickers, cultivation and pruning equipments.
  • Periodic disinfecting of the pruning tools while working on the tomato plants can help. Bleachers (such as Jik) and alcohol (70%) can be used. These can be wiped or sprayed on the pruning tools  
  • Avoid overhead and sprinkler irrigation methods; embrace drip and furrow irrigation which do not make the leaves wet. This is because the diseases are favored by warm, wet and humid conditions.
  • Use only certified seeds free from infections. In case of infected seeds, use hot water to treat
  • Regular and routine use of copper-based fungicides helps prevent the bacteria spores from germinating, thus acting as a preventative measure. It may also help in suppressing the diseases once already set in though it may not help much. Sulphur may also be used, but should not be applied at the flowering stage as it causes flower abortion. 

Here are some copper formulations that can be used:

  • Copper Oxychloride (green copper) e.g 

       Green Cop® 500WP https://www.agropests.co.ke/products?s=green%20cop 

       Isacop https://www.agropests.co.ke/products?s=isacop 

        Vitra https://www.agropests.co.ke/products?s=vitra  

Management of Septoria leaf spot (Septoria blight):

  • The above cultural practices for bacterial spot and specks can also be applied here.
  • Chemical control to be used here may include:

          Tebunaconazole eg Folicur https://www.agropests.co.ke/products?s=folicur , 

          Nativo https://www.agropests.co.ke/products?s=nativo , 

          Klassic https://www.agropests.co.ke/products?s=klassic    

          Uster Top https://agropests.co.ke/products?s=uster%20top        

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