Wheat Blast: A Disease Spreading by Intercontinental Jumps and Its Management Strategies

Abstract

Wheat blast (WB) caused by Magnaporthe oryzae pathotype Triticum (MoT) is an important fungal disease in tropical and subtropical wheat production regions. The disease was initially identified in Brazil in 1985, and it subsequently spread to some major wheat-producing areas of the country as well as several South American countries such as Bolivia, Paraguay, and Argentina. In recent years, WB has been introduced to Bangladesh and Zambia via international wheat trade, threatening wheat production in South Asia and Southern Africa with the possible further spreading in these two continents. Resistance source is mostly limited to 2NS carriers, which are being eroded by newly emerged MoT isolates, demonstrating an urgent need for identification and utilization of non-2NS resistance sources. Fungicides are also being heavily relied on to manage WB that resulted in increasing fungal resistance, which should be addressed by utilization of new fungicides or rotating different fungicides. Additionally, quarantine measures, cultural practices, non-fungicidal chemical treatment, disease forecasting, biocontrol etc., are also effective components of integrated WB management, which could be used in combination with varietal resistance and fungicides to obtain reasonable management of this disease.

Keywords: Intercontinental spread; Magnaporthe oryzae pathotype Triticum; disease spread; integrated disease management; wheat blast.

FIGURE 1

Wheat blast symptoms on different parts of the plant. (A) initial symptoms of blast in wheat field in a patch, (B) infected field showing silvery bleached spikes with green canopy, (C) typical partial or full bleached spikes in field, (D) a partially bleached spike with multiple points of infection, (E) dark-gray sporulation of the fungus MoT on the rachis, (F) infected awns show brown to whitish discoloration, (G) infected glumes show elliptical lesions with white to brown center and dark gray margins, (H) severely shriveled or wrinkled blast affected vs. healthy grains of wheat, (I) typical eye-shaped lesions with gray or whitish centers surrounded by dark brown margins on seedling leaf, (J) a severely damaged seedling field affected by MoT infection, (K) typical eye-shaped or elliptical lesions on a mature leaf, and (L) elliptical or elongated lesions on blast-affected stem having white centers surrounded by brown or blackish margins.

FIGURE 2

Blast and blast-like symptoms on wheat heads. (A) A typical blasted head having gray colored infection point, (B) symptoms of FHB showing superficial pink to orange masses of spores of F. graminearum with pink colored infection points, and (C) symptoms of spot blotch giving black discoloration on the infected spikelets because of B. sorokiniana.

FIGURE 3

(A) Pyriform two-septate hyaline to pale, gray-colored asexual conidia under compound microscope (magnification 400×) and (B) dark gray-colored colony of the fungus grown on potato dextrose agar (PDA) medium.

FIGURE 4

Spread of wheat blast in South America from 1985 to 2021.

FIGURE 5

Spread of wheat blast in Bangladesh from 2016 to 2021.

FIGURE 6

Spread of wheat blast in Zambia from 2018 to 2021.

FIGURE 7

Intercontinental spread of wheat blast attributed to grain trade.