Deadly Wheat Crop Infection Spreading Across Planet

[Guest Erwin Northoff]

Millions of wheat farmers, small and large, face this spreading and deadly crop infection. A virulent strain of wheat stem rust, Ug99, that sprung up in Africa in 1999 has now spread into Iran and threatens to spread into other wheat producing regions of Asia and eventually the entire world. In a nursery in Uganda, Africa in 1999, susceptible type stem rust pustules (collection designated Ug99) were found on wheat lines known to have the stem rust resistance gene Sr31, a gene for which no virulence had been reported previously anywhere in the world.

 

Countries east of Iran, like Afghanistan, India, Pakistan, Turkmenistan, Uzbekistan and Kazakhstan, all major wheat producers, are most threatened by the fungus and should be on high alert, FAO said.

 

It is estimated that as much as 80 percent of all wheat varieties planted in Asia and Africa are susceptible to the wheat stem rust (Puccinia graminis). The spores of wheat rust are mostly carried by wind over long distances and across continents.

 

“The detection of the wheat rust fungus in Iran is very worrisome,” said Shivaji Pandey, Director of FAO’s Plant Production and Protection Division.

 

“The fungus is spreading rapidly and could seriously lower wheat production in countries at direct risk. Affected countries and the international community have to ensure that the spread of the disease gets under control in order to reduce the risk to countries that are already hit by high food prices.”

 

The government of the Islamic Republic of Iran has informed FAO that the fungus has been detected in some localities in Broujerd and Hamedan in western Iran. Laboratory tests have confirmed the presence of the fungus. Iran said it will enhance its research capacity to face the new infection and develop new wheat varieties resistant to the disease.

 

 

COMMON NAME:

Stem rust, black rust

 

SCIENTIFIC NAME:

Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. E. Henn.

 

DISTRIBUTION:

Occurs worldwide wherever wheat is grown. It is most important where dews are frequent during and after heading and temperatures are warm, 18-30 C.

 

YIELD LOSS:

Losses are often severe (50 to 70%) over a large area and individual fields can be totally destroyed. Damage is greatest when the disease becomes severe before the grain is completely formed. In areas favorable for disease development, susceptible cultivars cannot be grown. Grain is shriveled due to the damage to the conducting tissue, resulting in less nutrient being transported to the grain. Severe disease can cause straw breakage, resulting in a loss of spikes with combine harvesting.

 

SYMPTOMS:

Uredinia generally appear as oval lesions on leaf sheaths, true stem, and spike. Uredinia can appear on the leaves if other diseases have not killed them. Uredinia are brick red in color and can be seen to rupture the host epidermis, on the leaves uredinia generally penetrate to sporulate on both surfaces. Infected areas are rough to the touch.

 

 

ENVIRONMENTAL CONDITIONS:

Stem rust is favored by hot days 25-30 C, mild nights 15-20 C with adequate moisture for night time dews. Wind can effectively disperse urediniospores over great distances. Rain is necessary for effective deposition of uredinospore involved in regional spore transport.

 

INOCULUM SOURCE AND INFECTION:

Aeciospores from Berberis vulgaris are currently rare, but historically it was an important source of inoculum in northern North America and Europe. Mycelium or uredinia on volunteer wheat, are the most important source of inoculum in tropical and subtropical climates. Windblown urediniospores are usually from earlier maturing wheat from the south in the northern hemisphere, or from the north in the southern hemisphere.

 

Urediniospores and aeciospore germinate when in contact with free water. Infection by penetration through the stoma. Penetration requires at least a low light intensity. Germination optimum is 18 C, latent period varies from 10 to 15 days in the field with temperatures of 15-30 C.

 

SURVIVAL:

Stem rust can survive as teliospores during winter when aeciospores are a major source of inoculum.

 

It generally survives as mycelium or uredinia on volunteer wheat during the non-wheat growing season. Uredinospore can be spread by wind into disease-free areas. Sporulating uredinia are active in tropical and some subtropical areas throughout the winter. Occasional dormant mycelium may survive beneath the snow pack in more northern temperate regions.

 

METHOD OF DISSEMINATION:

Urediniospores and aeciospores are wind borne. Teliospores remain with the straw.

 

HOST RANGE:

Stem rust is generally confined to Triticum species, although naturally infected plants of Secale cereale, Hordeum vulgaris, H. jubatum, H. pusillum, Elymus junceus occur. Many genera of the tribe Hordeae are infected when artificially inoculated. Other formae specialis of P. graminis attack many cereals and related grasses, and many species are susceptible to more than one formae specialis.

 

Erwin Northoff

Media Relations, FAO

erwin.northoff@fao.org

(+39) 06 570 53105

(+39) 348 252 3616