Wilt

Wilt is the drooping of leaves and stems when plant cells lose the water pressure (turgor) that keeps them firm. It’s a symptom, not a disease - wilt can result from several different causes that require different responses. Treating a wilted plant incorrectly wastes time and can make the problem worse.

Drought Wilt

The simplest cause: the plant doesn’t have enough water. Soil is dry; roots can’t supply water fast enough to replace what leaves are losing through transpiration. Plants recover after watering.

Temporary afternoon wilt in large-fruited tomatoes and squash is common even with adequate soil moisture. Transpiration demand on hot afternoons can briefly exceed water supply through roots. If plants recover by morning, this is normal and doesn’t require intervention.

Bacterial Wilt (Erwinia tracheiphila)

Bacterial wilt is the most destructive wilt disease of cucumbers and muskmelons, caused by Erwinia tracheiphila and spread exclusively by cucumber beetles (Acalymma vittatum and Diabrotica undecimpunctata). The bacteria overwinter in the beetles’ intestines and are deposited onto plants when beetles feed.

Progression: Rapid. A plant can go from appearing healthy to wilted and collapsed in 3-5 days as bacteria multiply and block xylem vessels throughout the plant.

Diagnosis: Cut a wilted stem near the soil, touch both cut ends together briefly, then slowly pull apart. In bacterial wilt, thin bacterial strands stretch between the two pieces (a sticky web of bacterial slime). This test distinguishes bacterial wilt from drought stress and Fusarium wilt.

Management: No cure once infected. Remove and discard the plant immediately to prevent beetles from moving the pathogen to healthy plants. Prevention through cucumber beetle management: row cover during the early season (remove when flowers open for pollination), or delayed planting to avoid peak early-season beetle populations.

Squash, pumpkins, and watermelons are resistant to bacterial wilt. Cucumbers and muskmelons are highly susceptible.

Fusarium Wilt

Fusarium oxysporum infects roots and colonizes the vascular system, blocking water movement. Wilt begins on one side of the plant or one branch and progresses to the whole plant over days to weeks. Unlike bacterial wilt, the progression is slower and cross-section of the stem shows brown vascular discoloration. Most common in warm soils (75-85°F).

Tomatoes, basil, watermelon, and many other crops have host-specific Fusarium wilt strains. See Fusarium for full detail.

Verticillium Wilt

Verticillium dahliae and V. albo-atrum cause similar vascular wilt symptoms in tomatoes, peppers, potatoes, strawberries, and eggplant. Progresses more slowly than Fusarium wilt. Vascular browning in the stem cross-section extends higher into the plant than Fusarium. More common in cool soils (60-75°F), unlike Fusarium which favors warm soil.

Tomato resistance to Verticillium wilt is coded “V” in variety descriptions. ‘Celebrity’ (VFFNT) carries Verticillium resistance.

Crown Rot and Root Rot

Various soilborne pathogens - Phytophthora spp., Pythium spp., Sclerotinia spp. - can cause root or crown rotting that produces wilting from the bottom of the plant up, similar in appearance to vascular wilt. The distinction: rot diseases show visible decay at the crown or root system when dug. Vascular wilts show clean (though discolored) vascular tissue without soft rot.

The Diagnostic Sequence

Before treating a wilted plant:

1. Check soil moisture first
2. Check for cucumber beetles if on cucumbers or melons
3. Dig and examine roots
4. Cut a stem and check for vascular discoloration or bacterial threads
5. Check for stem damage (borer entrance holes, mechanical injury, crown damage)

Wilting plus actively flowing sap = mechanical damage or stem-boring pest. Wilting plus dry soil = water. Wilting in well-watered plant plus brown vascular tissue = Fusarium or Verticillium. Wilting plus bacterial threads in stem = bacterial wilt.