Ground Cover Supplement : GC Supplement - Cereal foliar fungal diseases
15 Southern region TO SPRAY OR NOT TO SPRAY? Yellow spot of wheat is a constant companion of wheat growers across australia and controlling inoculum levels from season to season is the key to its control By dr andrew milgate AVOIDING YELLOW SPOT epidemics in the majority of seasons in southern New South Wales is simple -- do not sow susceptible varieties into stubbles containing moderate to high levels of inoculum. It really is that easy: control inoculum levels and you will control this disease. Along with resistant varieties, crop rotation is a very effective way to manage yellow spot, with inoculum loads reduced by up to 95 per cent in a single year where breakdown of wheat stubble occurs. While cheap fungicide options exist for yellow spot, it is important not to use sprays as the only strategy for managing the disease. Trials around Australia have demonstrated that fungicides containing the active ingredients propiconazole or tebuconazole are effective against the disease. It is important to remember that fungicides do not kill the yellow spot fungus living on dead leaf or stem tissue, so stubble inoculum and leaf lesions already present at the time of application can continue to produce spores. This means that when the fungicide wears off reinfection can occur rapidly if weather conditions are suitable. Similarly, seed and fertiliser fungicide treatments cannot control yellow spot in epidemic situations because they are unable to control the inoculum being produced on the stubble. EARLY-SEASON INFECTIONS Returns on fungicide inputs for yellow spot are often inconsistent and depend on the timing and size of the infection and seasonal conditions. While it is common to observe yellow spot in wheat-on-wheat crops in southern NSW during May to August (pre-GS31), particularly when susceptible varieties are sown, there is little evidence that such infections go on to reduce grain yields. This is because weather conditions at this time are usually unsuitable for the infection to survive until later crop growth stages. However, the situation is different in wet years and in high-rainfall regions of southern NSW, where early infections can establish the platform for the disease to cause yield losses. Under these conditions it has been demonstrated that a program of early and repeated spraying is required to minimise potential losses from the disease in susceptible varieties. When rainfall events are frequent, the ability to completely control the disease for an entire season is difficult due to continued spore production from the stubble and infected lower leaves and the fact that fungicides can only protect those parts of the plant with which they come into contact. The practical implication of this is that newly emerged or emerging leaves will be vulnerable to infection between fungicide applications and this will result in loss of some leaf area, even with multiple fungicide applications. MID-SEASON INFECTIONS Yellow spot epidemics commencing at GS31 to GS55 (August to September in southern Australia) are usually associated with weather events that promote mass spore release and infection. Rainfall outlooks will determine if spraying for yellow spot at this stage will be economical. However, if significant infection has occurred on the flag and flag-1 leaves then taking action to protect what green leaf remains may reduce further yield loss. If the diseased crop involves a susceptible variety sown into infected stubble (high pathogen load) then the risk of an epidemic is increased. Evidence from research trials shows that attempts to control yellow spot by spraying at GS31 and GS39 will only reap a return if weather conditions are conducive to continued disease development. In addition, spraying at GS31 and GS39 will only provide green leaf protection for three to four weeks, depending on the product used. While GS39 sprays give protection to the emerged flag leaf, any infection that has already occurred remains active with the potential to provide spores for additional infection once the fungicide becomes ineffective. LATE-SEASON INFECTIONS In relatively low-yielding environments there is little benefit from spraying for yellow spot at GS55 to GS85 (October to November). Crop moisture stress during these growth stages is usually well underway and yield is more likely to be limited by a lack of water than diseased leaf area. REDUCING YELLOW SPOT RISK Preparation is the key to avoiding problems with yellow spot. Growers should: ¢ choose varieties with higher levels of resistance; ¢ determine if yellow spot is present at significant levels before using a susceptible variety so that any stubble left in the paddock can be removed; ¢ burn, incorporate or graze stubble to reducing the inoculum load (and the need to spray); and ¢ rotate wheat with a non-host crop -- yellow spot is only a major pathogen of wheat so any other crop species is a non-host. □ grdc research code dan00177 more information: Dr Andrew Milgate, NSW DPI, 02 6938 1990, firstname.lastname@example.org Small tan-brown spots with yellow margins that become more elongated with age indicate the presence of yellow leaf spot. Yellowing of the leaf without lesions is not a symptom. PHOTO: GRDC PHOTO: DR HUGH WALLWORK Fungicide results against yellow spot are inconsistent and should not be the only strategy for managing the disease.
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