The ManicaPost
Wheat leaf rust is a fungal disease that affects wheat, barley and rye stems, leaves and grains. In temperate zones it is destructive on winter wheat. Infections can lead up to 20% yield loss exacerbated by dying leaves which fertilise the fungus. The pathogen is Puccinia rust fungus. Puccinia triticina causes ‘‘black rust’’, P.recondita causes ‘‘brown rust’’ and P.striiformis causes ‘‘yellow rust’’. It is the most prevalent of all the wheat rust diseases, occurring in most wheat- growing regions.
It causes serious epidemics in North America, Mexico and South America and is a devastating seasonal disease in India. All three types of Puccinia are heteroecious requiring two distinct and distantly related hosts (alternate hosts).
Rust and the similar smut are members of the class Pucciniomycetes, but rust is not normally a black powdery mass.
Leaf rust is generally found on the leaves, but occasionally is found on legumes and awns. Symptoms are circular or oval, orange pustules on the upper surface of infected leaves.
The spores within these pustules are easily dislodged and cover hands and clothing with an orange dust. As the wheat ripens, this orange stage turns into a new stage of the fungus, which is black, and often a mixture or orange and black stages will be intermingled on a leaf.
Plant breeders have tried to improve yield quantities in crops like wheat from the earliest times. In recent years, breeding for the resistance against disease proved to be as important for total wheat production as breeding for increase in yield. The use of a single resistance gene against various pests and diseases plays a major role in resistance breeding for cultivated crops. The earliest single resistance gene was identified as effective against yellow rust. Numerous single genes for leaf rust resistance have since been identified, the 47th genes prevent crop losses due to Puccinia recondite Rob. Ex Desm. f.sp. tritici infections, which can range from 5–15% depending on the stage of crop development.
Favourable weather conditions
The leaf rust fungus is an airborne pathogen whose spores are spread by wind over long distances. Once established in an area, the spores spread locally within fields as well as to nearby wheat fields. About every 7-10 days new rust pustules are formed. Weather plays a key role in the development of leaf rust. Moderate nights and warm (60°-80° F) days that create long dew periods in the wheat canopy are ideal for rust development. Under the right conditions, severe rusting of susceptible varieties occurs 30 to 40 days after initial rust development.
Management
Growing resistant varieties is the best and most economical method of minimizing rust losses. Irrigated wheat can be infected wherever it is grown.
Leaf rust is effectively controlled by the timely application of foliar fungicides. Fungicides currently registered for leaf rust control on wheat in include Quadris (azoxystrobin): Stratego (propiconazole + trifloxystrobin); PropiMax EC and Tilt (propiconazole) and the mancozeb-based products Manzate, Dithane, and Penncozeb.
Fungicide application may not always be cost effective if rust severity levels are low or if rust is slow to develop. Fungicide treatment should be considered only for susceptible varieties.
