Pineapple Guava

A proper thinning prune plus horticultural oil clears it.

Rows of small brown, tan, black, white, or waxy bumps along stems, twigs, leaf midribs, and leaf undersides that do not brush off but can be scraped loose with a fingernail. A sticky shine on the leaves - and on the floor, table, or path beneath the plant - that later turns black with sooty mold. Ants running steadily up and down the stems, since they farm and defend scale for that honeydew; often the first sign anything is wrong. Yellowing leaves, weak thin growth, and dieback of individual branches or canes that are heavily encrusted. On fruit crops, scale covers and sooty mold directly on the fruit. Check the leaf undersides and the shaded inner wood, which is where colonies build up unseen.

Horticultural oil is the main control: it works by smothering, so it has to coat the insects thoroughly - stems, twigs, and leaf undersides - and it usually needs repeating, because the waxy covering shields them and no single pass reaches everything. On deciduous woody plants a dormant-season oil smothers the overwintering stage while the leaves are out of the way, and timing a growing-season spray to the crawler stage, when the young are mobile and uncovered, is far more effective than spraying settled adults. On a small plant or a light infestation, scrub or wipe the scales off by hand, or dab them with a cotton swab of rubbing alcohol. Prune out and destroy badly encrusted wood rather than trying to save it. Wash off the sooty mold once the scale is gone - it lives on the honeydew, not on the plant, and stops once the supply does. Anything you spray must be labeled for the plant, and oils can burn delicate foliage, so test first.

Inspect. Look over leaf undersides and stems regularly - on a crop you pick, every harvest is a scouting chance - and check every new plant thoroughly before you buy it, plant it, or set it beside anything else, because scale almost always arrives on a plant rather than flying in. Quarantine new houseplants for a few weeks. Keep plants unstressed and not over-fertilized, since soft, forced growth is what populations build on. Prune for airflow and light to open up the shaded inner growth where scale hides, and prune out heavily infested wood as routine maintenance - on anything that gets a hard annual pruning, that alone removes most of any buildup. Leave the lady beetles, lacewings, and parasitic wasps alone, since they normally keep scale down to nothing, and keep ants off the plant where you can, because ants protect the colonies from those predators. Indoors, avoid hot, dry, still winter air.

Scale insects are sap-suckers that spend most of their lives immobile, fixed to a stem, twig, or leaf underside beneath a waxy or shell-like cover. They look far more like bumps, blisters, or crusty encrustations than like insects, which is why an infestation is usually well established before anyone notices it. Soft scales excrete large amounts of sticky honeydew that drips onto everything below and grows a black sooty mold - and those blackened leaves are often the first thing a gardener actually sees. Armored scales produce no honeydew but sit under a hard shield that protects them from most sprays. They spread as tiny mobile crawlers in the weeks after hatching, which is the only stage that moves and the only stage that is easy to kill. Populations build on stressed plants, on crowded shaded growth, and above all on plants kept indoors in dry, still air over winter.

Mainly a concern in Florida, Hawaii and the humid Gulf; a plant in California's dry summers or the Pacific Northwest is at much lower risk.

Bright yellow or orange powdery pustules on the youngest leaves, shoot tips, flower buds and developing fruit - the colour is distinctive and unlike ordinary leaf spot. Purple-tinged or brown lesions on slightly older leaves, sometimes with a yellow halo. New growth that puckers, twists and dies back at the tips. Repeated loss of each spring and summer flush, leaving a plant that never gets ahead. Symptoms concentrated on the soft new growth, since the fungus cannot infect mature hardened leaves.

Prune out and bag infected growth on a dry day, sealing it rather than composting or shredding it, since disturbing pustules releases clouds of spores. Protectant fungicides are used commercially and can hold a valuable plant, but they must be labeled for the crop and applied before infection to be worth anything. On a badly and repeatedly infected plant in a warm humid region, removal is often the honest answer. Where the disease is not yet known to occur, do not treat it quietly - report a suspected find to your state agriculture office or extension service, since early detection is the only thing that slows its spread.

Buy clean, inspected nursery stock and quarantine new myrtle-family plants for a few weeks before planting them near others. Avoid overhead watering and water in the morning so foliage dries. Space and prune for airflow, and do not push soft new growth with heavy nitrogen feeding, since only young tissue can be infected. Clean tools, gloves and boots between plants and especially between properties. Do not move cuttings, plants or prunings out of an area where the disease is present - most long-distance spread has been human-assisted.

A rust fungus that attacks plants in the myrtle family - eucalyptus, bottlebrush, guava, feijoa, allspice, myrtle and their relatives - and is established in Florida, Hawaii, California and parts of the Gulf coast. It infects soft new growth, producing vivid yellow to orange powdery pustules on young leaves, shoots, flower buds and fruit, followed by leaf distortion, dieback of the growing tips and, on repeatedly infected plants, progressive decline. Spores are microscopic and travel on wind, clothing, tools and infected nursery stock, which is how it has moved between regions. In some parts of the world it has devastated wild myrtle-family populations, so it is treated seriously by agriculture departments and is subject to reporting and movement rules in areas where it is not yet established.

Bagging is impractical on a shrub carrying hundreds of small fruit, so the control here is sanitation: because feijoas ripen by falling, daily collection of windfalls is already the harvest routine, and doing it without exception is what breaks the cycle.

Tiny puncture or sting marks on the skin of ripening fruit, sometimes with a weeping drop or a small sunken dimple around them. Fruit that softens, browns, or collapses on one side well before it should. Small white maggots in the flesh when the fruit is cut, and fruit that falls early and rots fast. Adult flies - larger than a housefly, with patterned wings - hovering around ripening fruit and resting on foliage in the morning. Windfalls under the tree squirming with larvae, which is where the next generation comes from.

There is no cure for fruit already infested - bin it sealed, and never leave it lying. Protein bait sprays are the standard commercial control and are what agriculture departments use in outbreaks; in a home garden, sanitation plus bagging does most of the work. If you find a fruit fly outside its normal range, or receive a quarantine notice, report it to your state agriculture department rather than dealing with it privately: several of these species are regulated, and it is illegal to move host fruit out of a declared quarantine area, including to friends and family in another county or state.

Bag the fruit. A paper, mesh, or organza bag slipped over each fruit once it reaches golf-ball size physically excludes the females and is the only home control that reliably works. Pick fruit at color break and finish ripening it indoors rather than letting it soften on the plant, which removes the window the flies need. Clear every fallen fruit at least weekly and dispose of it by bagging, deep burial, or solarizing in a sealed bag in the sun - never in an open compost heap, which just breeds more. Keep trees pruned small enough that you can actually reach and bag the crop. Hanging traps tell you when flies are active but will not protect a crop on their own.

Tephritid fruit flies lay their eggs under the skin of ripening fruit, and the maggots feed inside it, so the first sign is usually a fruit that looks fine and is full of larvae when cut. The Caribbean fruit fly is established across peninsular Florida and the Caribbean; Mediterranean and Oriental fruit flies are established in Hawaii and turn up in California, Texas and Florida as quarantine outbreaks. They prefer fruit that is beginning to soften and color, they build up on any fallen fruit left rotting on the ground, and populations run from late spring through fall. Because several of these species are regulated pests, an infestation is not only a garden problem - moving infested fruit is how quarantines start.