Gadenin日本語Get the appSudden collapse: bacterial wilt and fusarium
The most alarming thing that happens to a watermelon is a plant that looks perfect one evening and is flat on the ground the next afternoon, with the fruit half grown. That is a wilt, it is inside the plant's water-conducting tissue, and there is no treatment. Pull the plant, take it off the plot and do not compost it.
The two wilts behave differently and it is worth knowing which you have, because the prevention differs. Bacterial wilt is carried by cucumber beetles and hits any plant they have fed on. Fusarium lives in the soil, persists for years, and takes plants down in the same beds season after season. Cut a wilted stem near the base and look: bacterial wilt strings a fine sticky thread between the cut faces when you press them together and pull apart, and fusarium shows brown staining in the ring just under the skin.
| Symptom | Likely cause | Test or check | Action |
|---|---|---|---|
| Whole plant flat within a day or two | Bacterial wilt | Sticky thread between cut stem faces | Pull it; control beetles next year |
| One runner at a time yellows and dies | Fusarium wilt | Brown staining inside the stem base | Pull it; do not grow cucurbits there for years |
| Wilts at midday, recovers at night | Simply short of water | Dig down and feel the soil | Water deeply; it is not disease |
| Base of stem brown and soft | Collar rot from deep planting | Look at the soil line | Improve drainage; plant high next year |
A plant wilting at midday and standing up again by evening is thirsty, not diseased. A plant that stays down overnight after being watered is one of the wilts.
Anthracnose and the leaf diseases
Anthracnose is the leaf and fruit disease that most often ends a watermelon season early. It shows as dark, angular spots on the leaves that enlarge and run together, and then as sunken, water-soaked circular lesions on the fruit itself that make it inedible. It spreads in warm wet weather and moves fastest through a crowded, sprawling planting where the leaves stay wet.
Powdery mildew and downy mildew both turn up as well, mildew as white bloom on the upper leaf surfaces and downy mildew as angular yellow patches bounded by the veins with gray fuzz underneath. All three are easier to slow down than to stop, and every measure that helps is one you take before symptoms appear: spacing, mulch, air movement and not watering the leaves.
- Water the soil, never the leaves Use a soaker hose or water at the base. Overhead watering wets the foliage and splashes soil onto the lower leaves, which is exactly how these diseases start and spread.
- Mulch the whole bed Black plastic or a thick straw mulch stops rain splash carrying spores from the soil up onto the leaves. It is the highest value single measure against anthracnose.
- Stay out of a wet planting Do not walk, prune or harvest among watermelon vines while the foliage is wet. You carry spores on your clothes and hands from plant to plant.
- Remove the first affected leaves Cut out the earliest spotted leaves and bag them off the plot. Left on the ground they sporulate and become the source for the rest of the planting.
Beetles, aphids and borers
The first three weeks after planting are when insect damage does lasting harm. A young watermelon losing half its leaf area to beetles never really recovers, and cucumber beetles carry bacterial wilt as well as eating the plant, so keeping them off small plants is worth real effort. Floating row cover over the plants for that first period solves both problems at once.
The covers have to come off when flowering starts, without exception. A covered plant in flower gets no insect visits, sets nothing, and gardeners lose whole crops to a row cover that was left on out of caution. Take it off the day you see the first female flower and rely on inspection after that.
| Pest | When it appears | How to tell | What to do |
|---|---|---|---|
| Cucumber beetles | First month, and again late | Striped or spotted beetles chewing holes | Row cover on young plants; hand pick |
| Aphids | Early and in hot dry spells | Clusters under leaves, sticky sheen | Spray water underneath; silver mulch deters them |
| Squash vine borer | Midsummer | One runner wilts, sawdust at a hole in the stem | Slit the stem, remove the grub, bury the node |
| Spider mites | Hot dry weather | Pale stippling and fine webbing underneath | Spray water on the undersides regularly |
| Slugs and pillbugs | After rain, on fruit | Grazed patches on the underside of fruit | Lift fruit onto a tile or board |
Any insecticide applied while flowers are open costs you pollination for days afterwards. If you spray at all, spray in the evening once the flowers have closed.
Split fruit, hollow hearts and blossom end rot
Not everything that goes wrong with the fruit is a disease. A watermelon that splits open on the vine has almost always taken up water too fast after a dry spell, usually the first heavy rain after a fortnight without any. Steady watering right through to harvest is the prevention, and it is why letting the plant get bone dry and then soaking it is worse than either extreme on its own.
Hollow heart, a crack running through the center of the flesh, comes from uneven growth: a burst of rapid growth after a check, or too few seeds from partial pollination. Blossom end rot, a sunken leathery patch at the flower end, is a calcium delivery problem driven by irregular watering rather than by a shortage of calcium in the soil. All three point back to the same thing, which is watering consistency.
| Problem | What causes it | How to prevent it |
|---|---|---|
| Fruit splits on the vine | Heavy water after a dry spell | Water steadily; mulch to even out the soil |
| Hollow heart in the flesh | Uneven growth or too few seeds | Pollinate thoroughly; avoid drought then flood |
| Blossom end rot at the flower end | Irregular water disrupting calcium supply | Consistent moisture, mulch, no drought stress |
| Rotten patch on the underside | Fruit sitting on wet soil | Lift each fruit onto a tile or board |
| Bland, pale flesh at full size | Too much water late, or too little sun | Ease off water in the final ten days |
Inspection routine and the records that pay off
Watermelon rewards a fixed inspection habit more than most crops, because the things that matter appear on the undersides of leaves and the undersides of fruit, where you do not look by accident. Twice a week through the season is enough, and the whole round takes a few minutes on a small planting.
Keep the notes brief and positional. What is worth writing down is the date a symptom first appeared and which plant, in which part of the bed, showed it. A year later that tells you when to start the preventative work and which end of the bed to space more widely, and that is far more useful than a general memory that the plants got sick.
- Turn leaves over, twice a week Mites, aphids and downy mildew all start on the undersides. Looking only from above means finding them a fortnight later than you could have.
- Check the stem base every visit Look at the collar and the first foot of each runner for borer holes, sawdust and browning. This is where a plant is lost, and it is the part nobody looks at.
- Record first-sighting dates Note the day you first see any symptom and which plant it was on. Two weeks before that date next season is when the preventative work should start.
- Map which positions fail Record which end of the bed and which hills went down. Wilt and rot cluster in the same wet or sheltered spots year after year, and the map is what tells you not to plant there again.
Never compost a plant lost to wilt, and clear every scrap of vine and fruit at the end of the season. Cucurbit diseases overwinter in the debris left on the bed more reliably than anywhere else.