Careful consideration needed with yellow spot sprays
Author: Sarah Jeffrey | Date: 14 Sep 2016
All that is yellow in crops may not be as it seems and when it comes to making a decision to spray wheat for yellow spot - correct diagnosis of the disease is critical and can help growers save unnecessary expense on fungicide applications.
NSW Department of Primary Industries (DPI) senior plant pathologist, Steven Simpfendorfer, said yellow spot (Pyrenophora tritici-repentis) was often misdiagnosed and blames much of the confusion on its common name.
“In the past, the use of the word ‘yellow’ in naming the disease has meant some growers and advisers have assumed that any yellow discolouration in a wheat crop, whether it was associated with a spot or not, must be the leaf disease yellow spot and should be sprayed with a fungicide,” Dr Simpfendorfer said.
“We have seen fungicides applied to yellowing wheat crops, which are more characteristic of nitrogen deficiency, especially in waterlogged crops, or herbicide phytotoxicity, which can be caused by herbicide tank mixes and when applications coincide with frost events.
“Given the tendency for misdiagnosis, if you think you have yellow spot there are some simple checks that should be undertaken before applying any fungicide.
“First, check wheat stubble in the paddock for black fruiting bodies (pseudothecia), then check the current wheat crop - if lesions or spots have a small yellow margin with a tan centre, yellow spot leaf disease could be present.
“Yellow spot characteristically has spots randomly distributed across the length of leaves and does not concentrate only at the leaf tips. Within leaves on an individual tiller there will also be a characteristic distribution with more and larger lesions on the lowest leaf and fewer and smaller spots on leaves as you progress up the canopy.
“If you aren’t confident in your diagnosis, get a second opinion from a plant pathologist.”
It’s important to identify the source of inoculum, whether it’s from stubble in the wheat paddock or a neighbouring paddock, to better inform management decisions.
Stubble carrying high inoculum loads within the same paddock has a greater risk of causing repeated infection events than stubble further away as yellow spot spores have a limited spread of up to about 100 metres in wind.
Information on the identification and management of yellow spot can be found in the Grains Research and Development Corporation (GRDC) northern region factsheet available from the GRDC website or by following this link. There are also a number of videos on yellow spot on the GRDC’s YouTube channel.
Yellow spot is a stubble-borne wheat disease which affects many Australian wheat varieties and is known as yellow leaf spot in Western Australia and tan spot in other countries.
It is a necrotrophic pathogen that feeds on dead plant cells which the fungus kills by producing a toxin, giving infected lesions on leaves a thin yellow outline.
In NSW and Queensland yellow spot is commonly a problem for susceptible varieties in wheat-on-wheat rotations, at seedling growth stages and through tillering.
The GRDC Grower Solutions Groups Grain Orana Alliance (GOA) and Northern Grower Alliance (NGA) have undertaken research into the effectiveness and economics of fungicide applications for yellow spot at various stages of crop growth.
The trials found economic benefits from applying fungicides to suppress yellow spot were negligible, particularly with early fungicide applications during the seedling stages.
This is primarily because during tillering the key yield contributing top three leaves have not emerged and therefore will not be protected by a fungicide application; infection from ascospores on wheat stubble can occur over a protracted period as the fruiting bodies do not all mature at the same time; and any inoculum reduction gained through limiting the number of early leaf infections in the lower canopy is quickly swamped by the mass of ascospores released from continually maturing fruiting bodies on the stubble throughout the season, when wet weather and rain events support their development.
Reports from the trials are available in GRDC Update papers at the following:
Dr Simpfendorfer said fungicides were generally less effective against yellow spot compared with what growers normally achieved with rust management.
“This is because all fungicides have very limited curative activity against yellow spot and never kill the yellow spot fungus within infected leaves,” he said.
“Fungicides are potentially more effective used in a preventative strategy prior to rainfall events.
“To achieve an economic return, fungicide management strategies need to be based around protecting the green-leaf area in the top three leaves.
“An initial fungicide application at GS30-32, followed by a second application at full flag-leaf emergence, GS39, if the season warrants, is likely to provide the best management of yellow spot in susceptible wheat varieties.”
Effective long term management of yellow spot requires careful pre-season planning. Growers are advised to select a variety with some level of resistance to yellow spot, avoid sowing wheat-on-wheat, or if sowing wheat-on-wheat be aware of the presence of yellow spot inoculum on stubble and consider appropriate stubble management practices such as a late autumn burn.
Good agronomic management in areas like soil and crop nutrition can also be an effective means of limiting the disease’s yield and economic impact.
Anne Brook, NSW DPI media officer
0477 358 305
Sarah Jeffrey, Senior Consultant Cox Inall Communications