Blister blight, the disease that attacks the crop
Exobasidium vexans infects only young, still-expanding tea leaves, which is precisely the tissue that is harvested. Infection produces small translucent spots that develop into raised blisters on the underside of the leaf, and heavily affected shoots are unusable — the damaged tissue is brittle, the shoot may snap, and infected material carried into the factory degrades the batch. It needs prolonged leaf wetness and cool conditions to establish, so it is a disease of high-elevation and monsoon-season tea: severe in the Sri Lankan high country, in the Nilgiris, in Darjeeling and in north-east India during the wet months, and largely absent from warm dry regions. The geography is worth dwelling on because it inverts the usual assumption. High-elevation fields are widely taken to be cleaner growing environments, and for insects that is broadly true. For this particular disease the opposite holds: the cool, persistently misty conditions that make high-grown tea desirable are exactly the conditions the pathogen requires, so the most valuable fields in several countries carry the heaviest disease pressure.
Why blister blight is hard to manage
The economics are awkward. The disease cycles fast in favourable weather, so protection has to be maintained continuously through a wet season rather than applied once, and it has to be maintained on a crop being harvested every week or two. That combination — repeated applications on a continuously harvested crop — is exactly the situation in which residue problems arise, and it is one of the standing tensions between plant protection and market residue limits in high-elevation origins. Cultural measures help at the margins: wider spacing and reduced shade improve air movement and shorten the period leaves stay wet, at some cost to the other things shade was doing. Plucking round length matters as well, and in a direction that surprises people. Picking more frequently removes young infected tissue from the field before the fungus completes its cycle on it, so a shortened round during a wet spell functions as a control measure in its own right. It requires labour to be available exactly when the weather is worst, which is the practical reason it is often not done.
The leaf and stem diseases
Beyond blister blight, a group of fungal diseases attacks mature leaf and wood. Grey blight and brown blight produce spreading necrotic patches on older leaves, usually entering through damage — which makes them a downstream consequence of hail, of insect feeding or of careless plucking. Stem canker and die-back attack wounded wood, and pruning cuts are the obvious entry point, which is why pruning technique and timing matter beyond their effect on growth. None of these individually accounts for large losses; collectively, in a poorly maintained field, they are why the frame fills with dead wood. The pattern across this group is that they are opportunists. Each of them needs an opening — a wound, a weakened plant, a stressed field after drought — and healthy vigorous tea largely resists them. That makes them useful as indicators: an outbreak of leaf blight across a block is usually reporting something about the block’s general condition rather than about the fungus, and treating the symptom without asking why the field was open to it is how the same outbreak recurs.
Root diseases, and why they are the serious ones
The genuinely destructive diseases of tea are root and collar rots caused by soil fungi, including Armillaria and the red and brown root diseases. They spread through soil contact between root systems, so they move outward from a focus and produce the characteristic expanding patch of dead and dying bushes seen in old fields. By the time above-ground symptoms appear — yellowing, wilting, sudden collapse — the root system is already largely gone. Because the pathogen persists in the ground and in buried wood, replanting straight into an infected patch usually loses the new plants too. This is one of the few situations in tea where a grower is dealing with something genuinely invisible. A patch that looks healthy today may already have the fungus moving through it, and the first above-ground sign appears only when a plant can no longer supply itself with water. Managers who know their fields well track the edges of known patches year on year, because the rate of spread is the only early information available.
The connection to how land was cleared
A recurring pattern is that root disease is worst where tea was planted directly onto cleared forest land without removing the stumps and large roots, because those provide a food base on which the fungus builds up before moving to the tea. This is a decision taken once, at establishment, whose consequences appear twenty years later and cannot be undone. Managing an infected area means isolating it — trenching to break root contact — removing infected material, and leaving the ground under a non-host crop for a period before replanting. All of that costs years of production from that block. It also explains an otherwise puzzling piece of practice: why careful estates spend money removing old stumps and large roots when clearing land they are about to plant, decades before anyone involved will see the consequence. That expenditure is the cheapest point in the whole cycle at which the problem can be addressed, and skipping it is one of the clearest cases in tea of a saving that is repaid with interest.
Resistance, and what it can and cannot do
Breeding programmes select for resistance to blister blight in particular, and some widely planted clones are markedly less affected than others, which is one of the practical arguments for planting several clones in blocks rather than one across an estate. Root diseases are harder to breed against and are managed culturally rather than genetically. The general lesson is the same as elsewhere in tea cultivation: a great deal of what determines a field’s disease burden was decided at planting, and the operations available afterwards are containment rather than reversal. It is worth adding what this means for the reader rather than the grower. Disease pressure is one of the main reasons a producer’s input use looks the way it does, and it varies enormously between a dry lowland field and a misty high one. Judging two producers by the same standard without knowing what each is up against produces conclusions that are confident and wrong.