Yellow Spot is arguably the most economically damaging wheat disease in Australia, decreasing yields by approximately $212 million dollars per annum, an average of $18/ha (Murray and Brennan, 2009). In hard hits areas, losses can exceed $30/ha.
Yellow Spot is caused by a fungus (Pyrenophora tritici-repentis) and is prevalent in wheat crops world-wide. Overseas the disease is known as Tan Spot.
The fungus is necrotrophic, meaning it feeds on dead plant tissue. This is achieved through the secretion of effectors (host-selective toxins) that kill wheat cells. As the cells die, the leaf turns yellow and brown, resulting in the characteristic spots (lesions).
Reduced green leaf area impacts the ability of the plant to photosynthesise, which can result in decreased grain yields. Yield losses of up to 30% have been recorded in susceptible varieties.
The fungus survives in crop stubble for up to two years. In the autumn/winter sexual fruiting bodies (pseudothecia) can be seen as raised black, pinhead sized projections around the stubble stem nodes.
During wet conditions, sexual spores (ascospores) are released from the fruiting bodies and infect young plants adjacent to the stubble. This is the primary infection.
A secondary infection will then follow via the production of asexual spores (conidia) that are dispersed by rain and carried by wind to infect the crop canopy and beyond.
Although losses exceeding 15% are unlikely, exceptionally wet seasons can exacerbate the severity of infection.