Head infection: the things to watch
By Dr Colin Wellings*
University of Sydney Plant Breeding Institute Cobbitty Stripe rust has been epidemic in eastern Australia in 2003. Chemical control has been used extensively, and has provided good protection in many situations.
However, infection of the head has occurred, even when disease levels in the crop canopy have been low. This article provides some background to head infection with comments on yield and quality implications.
Symptoms: Bleached, discoloured florets with faint evidence of yellow rust spores can be seen from first inspection of suspect heads (see plate 1). These symptoms may be initially confused with other diseases, such as Fusarium head scab. Peeling back glumes from affected florets will reveal abundant yellow rust spores being produced on the inside of the floret adjacent to the developing seed (see plate 2).
Disease Cycle: Spores germinate and infect florets from heading to flowering, with symptoms developing over the following 10 to 20 days. Infection does not occur after flowering. Although spores may adhere to seed, they are not expected to survive for more than a few days. Stripe rust is not a seed-borne disease.
Variety response: In general, varieties resistant in the canopy will be resistant in the head; conversely, those susceptible in the foliage will tend to support a high pathogen load and head infection may become severe.
However, there are many situations where severe head infection has occurred despite moderate to low levels of rust in the crop canopy. This is due to high levels of inoculum that may be generated from crops adjacent to or nearby the unexpected head infections. Yield effects: It can be expected that head infection will produce shrivelled grain, although the extent of this will depend on how early the infection established in a particular floret and how many florets become infected. Screenings will be expected to increase with severe infections. Seed staining has been reported in severe cases of head infection. Note that the pathogen does not produce toxins that would prevent the use of downgraded seed as stock feed.
Chemical control: Chemical control of head infection is not considered to be effective, despite the excellent levels of disease reduction that can be achieved with foliar sprays. This is due to poor, if any, translocation of fungicide from the flag leaf to the head, and poor coverage of chemical targeted for head control only.
*Dr Wellings is on secondment from NSW Agriculture.