Bacterial wilt on cucumber — symptoms and treatment

Severity: high

What is Bacterial wilt

Bacterial wilt is caused by Erwinia tracheiphila and spread by cucumber beetles. The bacteria live and multiply inside the plant's water-conducting vessels, plugging them until water can no longer reach the leaves. Infected plants wilt and die rapidly. It is one of the most damaging problems a cucumber crop can face, because by the time a plant shows symptoms the bacteria are already established throughout its vascular system and no spray will cure it.

Symptoms

The first sign is usually a single leaf, or part of one runner, that goes limp while the rest of the plant still looks healthy. The wilting often looks like drought stress at first, and a plant may even recover overnight and wilt again the next day. Within a short time the collapse spreads leaf by leaf and runner by runner until the whole plant goes down. The diagnostic test is in the stem: cut through a wilted stem near the base and the cut surface exudes a sticky bacterial ooze. Pressing the two cut halves together and drawing them slowly apart will often pull that ooze out into fine threads.

• Early: sudden wilting of individual leaves or single runners, sometimes recovering temporarily
• Early: cut stems exude sticky bacterial ooze that strings out between the cut surfaces
• Advanced: wilting of the entire plant and rapid plant death

Bacterial wilt is easy to confuse with drought stress, root damage or squash vine borer injury. The sticky ooze from a cut stem is what separates it — thirsty plants and borer-damaged plants do not produce it.

Causes and conditions

The bacteria do not survive well on their own in soil or debris. They overwinter inside cucumber beetles and are carried from plant to plant in the beetles' mouthparts and droppings. Beetles feed on leaves, deposit bacteria into the fresh wounds, and the infection enters the vascular system directly. This is why bacterial wilt is fundamentally a beetle problem: wherever cucumber beetles are active, the disease follows them. Young plants are especially vulnerable, since a single infection at the seedling stage can take out the whole plant before it ever sets fruit.

Treatment

Row covers and trap crops — cultural

Use floating row covers until flowering. Remove and destroy wilted plants immediately. Plant trap crops. Removing infected plants matters because they act as a reservoir the beetles feed on and carry away from. Covers must come off at flowering so pollinators can reach the blooms.

Imidacloprid (beetle control) — chemical

Control cucumber beetles which spread the bacteria. Apply systemic insecticide at transplanting. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Cover young plants with floating row covers early in the season, before beetles find them
• Scout regularly for cucumber beetles and act on the beetles rather than waiting for wilt
• Pull and destroy wilted plants as soon as you find them — do not compost them
• Use trap crops to draw beetles away from the main planting
• Grow varieties with less beetle appeal where they are available locally

Frequently asked questions

Can a wilted plant recover? No. Once the bacteria are inside the vascular system there is no cure, and the plant will die. A plant that perks up overnight is not recovering — it is in the early stage. Remove it.

Is it contagious to other plants? Yes, but not by touch or by splash. It moves only when cucumber beetles carry it from an infected plant to a healthy one, so controlling the beetles is what stops the spread.

When should I treat? Before symptoms appear. Row covers go on early, and systemic beetle control goes in at transplanting. Spraying a wilting plant accomplishes nothing.