Cucumber spots...

Asked June 29, 2019, 9:52 PM EDT

I have tried splendid, neem oil and copper. I am worried this is mosaic though I didn’t have aphids this year. Linus

Hennepin County Minnesota

1 Response

Cucumbers are susceptible to several leaf spot diseases. It's difficult to determine which is present by viewing photos. Sometimes laboratory analysis is needed to accurately identify the disease.

Please compare affected leaves on your plants to the following photos and see what you think. Based on our review, we suspect angular leaf spot but we can't be sure.

If you think angular leaf spot is present the following information from the University of Massachusetts may be useful:

Angular leaf spot (Psuedomonas syringae pv. lachrymans)

Angular leaf spot can affect all cucurbits, but cucumbers are most commonly affected. It is caused by the bacterium Pseudomonas syringae pv. lachrymans. This disease is usually among the first to show up because it is seed-borne. It will start to appear in the early to mid-season. Small, round water-soaked spots appear on leaf tissue, and expand until they are confined by veins, giving them the characteristic angular look. Under moist conditions a milky white exudate containing bacterial cells may ooze out of the lesion on the lower leaf surface. These wetlooking spots will dry out and turn yellow-brown or the dead tissue may fall out leaving a “shot-hole” appearance. Yellowing of the leaf between lesions may occur where disease severity is high. Similarly, water-soaked spots may appear on stems and petioles, drying out to form a whitish crust. Spots can also appear on fruit, where they are tiny and water-soaked but dry to form whitish, chalky, spots. These spots cause internal decay of fruit, and fruit that is infected early may be deformed. Affected plants will grow poorly, produce less fruit, and affected fruit is unmarketable.

As with other bacterial diseases, outbreaks of angular leaf spot are often initiated from infected seed. Bacteria proliferate in warm, moist weather and are spread from plant to plant by water, commonly in the form of splashing rain or runoff, as well as by insects or workers moving through the field.

Cultural controls:

  • Use resistant varieties.
  • Use drip irrigation to reduce spread of bacteria by overhead irrigation.
  • Don’t work in wet fields or work in clean sections of the field first and infected sections last to avoid spreading the disease to unaffected areas or to new plantings.
  • If you catch the disease early, before it is widespread and severe, copper may be effective in reducing its spread.
  • Till in residues quickly after harvest to get that infected tissue breaking down quickly. Bacteria survive on residues as long as it is present, up to two years.