Gadenin日本語Get the appFruit fly is the whole game
Guava is among the most attractive host fruits there is for fruit fly. Where these flies are present, an unprotected guava crop is routinely a total loss, and the frustrating part is that the tree looks fine the whole way through. The female stings the young fruit, laying eggs under the skin through a puncture barely visible as a small dark dot with a slight dimple. The fruit continues to swell and color normally. You pick it at the right moment, put it on the tray, and three days later the flesh has turned to brown mush with maggots in it.
Because the damage is invisible until after harvest, growers commonly go through two or three seasons blaming their ripening technique, their watering, or the weather before they work out what is actually happening. If your guavas repeatedly collapse from the inside within days of picking, the cause is fruit fly and no change to your ripening routine will fix it.
Traps help you know the flies are present and reduce numbers a little, but they do not protect a crop. Sprays are poorly timed against a pest that lays inside the fruit. Bagging is the answer, and it is close to a complete answer: bagged fruit comes through clean at rates well above ninety percent.
Bagging, step by step
Bagging is tedious once a year and then finished. It is also the reason the pruning advice insists on keeping the tree at a height you can reach, because you must handle every fruit individually.
- Bag at marble size Wait until the fruit is about three quarters of an inch to an inch (2 to 2.5 cm) across, which is roughly two to three weeks after petal fall. Earlier than that and you knock off fruit that has not set properly; later and the flies have already laid.
- Thin as you go Where several fruit are clustered, keep the largest one or two and remove the rest. Fewer, larger fruit is the better trade, and you have fewer bags to fit. Take off any fruit that already shows a puncture dot.
- Use bags that breathe Paper fruit bags, fine mesh sleeves or organza drawstring bags all work. Plain plastic does not; it traps condensation and rots the fruit. The bag must be large enough that the fruit does not press against the fabric, since a fly can lay through material held tight against the skin.
- Close the neck properly Gather the bag around the stalk and tie or twist it shut so there is no gap at the top. Most bagging failures are a loose neck rather than a bad bag.
- Check for the color break through the bag Mesh and organza let you see the fruit. With paper bags, open one from each part of the tree from time to time to judge the timing, and leave the rest closed until you pick.
Clear up every fallen guava, every day, through the whole season. Fallen fruit is where the next generation of flies completes its life cycle, and a single overlooked windfall under the tree undoes a lot of bagging.
The other pests worth knowing
After fruit fly the list is short and none of it is season-ending. Most of it is dealt with by keeping the center of the bush open, clearing fallen leaves and fruit, and looking at the undersides of the leaves once a week during the growing season.
| Problem | What you see | When | What to do |
|---|---|---|---|
| Scale insects | Brown or white raised bumps on stems and leaf undersides, sticky leaves, sooty black film | Any time, worst on indoor and potted trees | Wipe off by hand or with a soft brush, then treat with horticultural oil; repeat in two weeks |
| Mealybug | White cottony masses in leaf joints and around fruit stalks | Warm weather and on overwintered indoor trees | Spot-treat with alcohol on a cotton bud, then oil spray; check where branches meet the trunk |
| Whitefly | Small white insects lifting in a cloud when the branch is moved | Late in the growing season, and indoors | Rarely serious outdoors; on a potted tree use sticky traps and a soap spray on leaf undersides |
| Birds | Pecked holes in coloring fruit, whole fruit gone | As fruit approaches the color break | Bagging solves this too; otherwise net the tree, or simply pick earlier |
| Root rot | Sudden wilting of a whole branch or the whole tree, soil constantly wet | After a wet cool spell or in a poorly drained pot | Usually fatal once wilting starts; prevention is drainage, and mounding at planting |
Leaves, spots and yellowing
Anthracnose is the fungal disease you will meet most often on guava. It shows as dark sunken spots on the fruit and dieback at the tips of young shoots, and it flares in warm humid weather with poor airflow. It is a management problem rather than a spraying problem: prune to keep the center open, remove and destroy affected fruit and shoot tips rather than dropping them under the tree, and avoid wetting the foliage when you water. Leaf spot behaves the same way and rarely does enough damage to be worth treating on its own.
Yellow leaves need separating into three kinds. Pale new leaves with green veins are a micronutrient problem, usually iron or zinc, and are common on alkaline soil and in old potting mix; correct them with a foliar trace-element feed. Uniformly yellow lower leaves on a tree with permanently wet soil are waterlogging, and the fix is drainage, not fertilizer. A general leaf drop within two weeks of moving a potted tree indoors or outdoors is simply the tree adjusting to a change in light, and it will recover without intervention.
When to give up on a tree
Guava is cheap to replace and fast to come into bearing, which changes the calculation on a struggling plant. A tree that has lost its top to frost but has live green wood at the base will regrow and crop within two seasons, so cut it back to live wood and let it go. A tree wilting from root rot in soggy ground will not recover, and replacing it on a raised mound is a better use of the season than nursing it.
Keep a simple problem log with dates: when you first saw scale, when you bagged, how many bagged fruit came through clean against how many unbagged ones did, and the date any disease first appeared. The bagged-versus-unbagged comparison in particular is worth doing deliberately in your first protected season, because the numbers are usually so lopsided that you will never skip bagging again. First-appearance dates for scale and anthracnose let you start looking a week earlier next year, which is when both are still easy to stop.