CUCUMBER - BACTERIA

Host range: All cultivated species from the family Cucurbitaceae can be infected but cucumber is probably the most heavily affected. It is not an emerging pest, however it is one of the most serious diseases of cucumbers and causes severe losses for farmers.

Occurrence and importance: Under certain conditions it can result in significant (sporadic) disease outbreaks.

Symptoms and confusable diseases: Symptoms occur on leaves in the form of angular spots restricted by veins. The spots are water soaked with possible occurrence of milky drops of slime at the beginning and necrotized later. Finally, centres of spots disintegrate giving a tattered appearance to leaves. Infections on stems, petioles and fruits develop into water soaked spots later covered with white crust. Early infected fruits can be curved or deformed later. Secondary rots develop on the infected fruits. Occasionally, early symptoms on leaves could be confused with downy mildew but in this case spots are yellow, without drops of slime and not water-soaked, grey to black growth on the lower side of the spot can occur and later necrotized tissue does not fall away.

Disease cycle: The pathogen overwinters in plant debris for up to 2 years and is also seed-borne. Mechanical injury is the only point of its entry into the plant. The bacteria easily spread under warm, wet weather by wind-blown rain and touching the plants during cultivation, harvest etc. Optimal temperature for disease development ranges between 24-28°C. On the contrary, two weeks of dry weather stop the infection. Excessive doses of nitrogen make plants more susceptible to infection.

Control:

A complex system of control measures should be applied:

• fruits from which seeds are to be obtained should originate from healthy stands only and be thoroughly fermented;
• disinfection of seeds - 20 minutes with water at 50 ⁰C containing various acidic organic chemicals;
• crop rotation, a two-year rotation behind crops other than cucurbits and cultivating the soil when it is dry will decrease the ability of the bacterium to survive to infect upcoming cucurbit crops;
• resistant varieties;
• limit the use of overhead irrigation and excessive levels of nitrogen fertilization;
• in the greenhouse, maintain a relative humidity between 80 – 90 %;
• use drip irrigation instead of overhead sprinklers if possible, if watering by hand, water at the base of the plant where the vine meets the soil;
• rouging of infected plants;
• avoid touching the plants when they are wet (transplanting, cultivation, pruning, harvest);
• if bacterial canker was present on field plough under the plant residues into soil to speed up its decomposition;
• use of copper fungicides is helpful
• remove and destroy infected fruit and vines at the end of the season in small gardens
• bacteriophage-based biocontrol could provide a sustainable and natural alternative approach to combat bacterial pathogens (https://ami-journals.onlinelibrary.wiley.com/doi/10.1111/1751-7915.13585).