Wintergreen

Pale yellow leaves with green veins (interveinal chlorosis), especially on new growth, plus weak growth, sparse flowering, and muted fall color on neutral or alkaline soils.

Acidify the soil with elemental sulfur and an acid-forming (heath/blueberry-type) regimen, apply chelated iron as a stopgap, and mulch acidically; the durable fix is siting it in genuinely acidic ground.

Plant sourwood only in acidic, well-drained, humus-rich soil; use an acidic mulch (pine straw, shredded leaves), avoid liming near it, and test soil pH before planting.

Sourwood is a heath-family tree that needs acidic soil; on neutral or alkaline ground it can't take up iron and other nutrients properly, causing yellowing leaves (with green veins), poor growth, weak flowering, and dull fall color - the most common reason a planted sourwood fails to thrive.

A plant of the cool, shaded, moist forest floor, so it browns and declines in too much sun or dry soil - worst in hot regions and open sites. Part-to-full shade, consistently moist acidic soil, and a pine-needle or leaf-litter mulch to keep the roots cool.

Browning that starts at the leaf edges and tips and spreads inward, with a fairly sharp line between the crisp brown tissue and the green that is still healthy. Leaves and fronds going dry, papery, and crisp rather than soft, yellow, or spotted - that is what separates this from a fungal problem. Damage worst on the side of the plant facing the sun or the prevailing wind and on the most exposed upper leaves, while the shaded interior stays fine. Symptoms appearing sharply after a heat wave, a dry spell, or a bright windy week in winter. Whole leaves or fronds shed early as the plant cuts its losses.

Water deeply, mulch, and improve the shade - that is the whole treatment, because there is nothing here to kill. Leaves that are already scorched will not recover, so trim off badly browned foliage or fronds for appearance if you want to; the crown and roots are usually unharmed and push out fresh growth once conditions improve. On a plant that scorches badly every year the answer is to move it rather than to keep watering harder, because the siting is what is wrong. Some plants respond well to being cut back hard, which brings a clean flush of new foliage. And on plants that naturally die back or brown off late in the season, scorching at the end of a dry summer is normal and needs nothing at all.

Site the plant where it actually wants to be. This is a placement problem far more often than a watering problem, and a shade plant in hot afternoon sun will scorch every summer however much you water it. Give shade and woodland plants part-to-full shade and shelter from hot afternoon sun and drying wind. Keep the root zone consistently moist and mulch it well with leaf litter or another organic mulch, which holds water and keeps the roots cool - the single most useful routine step. Water deeply and thoroughly during dry spells rather than little and often, so moisture reaches the whole root zone. Pay particular attention through the first year after planting, when the root system is small and cannot keep up with the leaves. On evergreens in cold regions, make sure the plant goes into winter well watered, and keep it out of bright, windy, exposed positions.

Leaf scorch is not a disease and there is nothing to spray for it. It is what happens when a leaf loses water faster than the roots can replace it: the tissue farthest from the veins - the margins and the tips - dries out first and turns brown and papery, and the browning works inward while the rest of the leaf stays green. Hot sun, dry soil, drying wind, heat reflected off paving or a wall, a small or damaged root system, and a plant sited somewhere sunnier or drier than it wants are the usual causes, singly or together. It also happens in winter on evergreens, when bright sun and wind pull water out of the leaves while the ground is frozen and the roots cannot resupply. Shade and woodland plants, shallow-rooted plants, and anything recently planted scorch most easily.