Rice blast

What you are looking at

Grey or whitish-centred lesions shaped like a diamond or a spindle, edged in brown or reddish-brown, scattered across the leaves of a rice plant. In a bad attack these coalesce until whole leaves look scorched from a distance, though the individual lesion shape is what separates this from a simple environmental leaf browning.

The more serious form of the disease attacks the neck of the panicle rather than the leaves. A blackened, constricted rot girdles the stem just below the developing head, called neck blast, and it kills the whole panicle while it is still green, before any grain can fill. A field can look green and healthy from a distance while a large share of its panicles have already been killed this way. Lesions can also appear on leaf collars, nodes and stems, weakening the plant at those points, and on the seed itself, which is one of the ways the fungus carries over into the next season.

The most damaging disease of the world’s most important staple crop

Rice blast is present wherever rice is grown, including documented outbreaks in the United States, Europe and South America, though it does its worst damage across the major rice-growing regions of Asia, where warm, humid conditions, dense planting and continuous cropping all favour it. Estimates put global losses at 10 to 30 percent of the rice crop most years, enough lost grain to have fed tens of millions of people, with the economic cost running into the tens of billions of dollars. Because rice is the staple food for more than half the world’s population, this single disease is counted among the most consequential plant pathogens on earth.

What makes it difficult to control long-term is how fast the fungus adapts. A resistance gene bred into a new variety can be overcome by the local blast population within as little as three to five years, which is why regional guidance on which varieties currently hold up matters more than a single resistant variety chosen once and relied on indefinitely.

What to do about it

Grow a variety with resistance suited to the blast races present locally, and follow current regional guidance rather than assuming a resistant variety stays effective indefinitely.

Avoid heavy nitrogen, particularly late in the season, since lush growth is markedly more susceptible.

Start from certified, disease-free seed, and destroy or bury crop debris after harvest rather than leaving it on the surface.

Keep standing water on the crop where the growing system allows it, since continuous flooding suppresses the disease compared with intermittently dry conditions, and space plants at the recommended density rather than crowding them, since a dense, humid canopy favours infection.

Reach for fungicide only where a resistant variety is not available or cultural measures have already failed to hold a high-pressure season back; timed to booting and heading, the stages when neck blast does the most damage, it works as a last-resort supplement to resistant varieties rather than a substitute for them.

What it gets mistaken for

Leaf scorch and other physiological leaf browning follow a simple environmental cause, such as drought, salt or nutrient stress, and spread as a single browning margin from the leaf tip and edge inward. Rice blast produces individually shaped, scattered diamond lesions with a grey centre rather than one advancing scorched margin, and it is the neck rot that kills a healthy-looking panicle outright that has no equivalent in a straightforward physiological scorch.