There has been a tendency in some overseas countries, as well as in some Australian circles, to heavily base crop disease control on fungicides.
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However, research supports that while fungicides are an important crop disease control weapon, where possible it is best to utilise varieties with best known genetic resistance to likely problem diseases.
Fungicides can certainly help control important crop diseases, like Ascochyta in chickpeas, chocolate spot in faba beans, the three rusts in wheat, and barley diseases like net form of net blotch and powdery mildew.
It's worth noting there are several more diseases, of these and other crops, with fungicide control options.
Like herbicide control for weeds, dependence on fungicides for disease control also has the risk of diseases developing resistance to them.
This is already widely occurring overseas and in Australia.
New products are being identified to replace those with resistance development but also like it is for herbicides and weeds, these new products all too commonly run the risk of developing resistance to diseases.
Some diseases, like crown rot in wheat and barley, while having some varieties with useful tolerance, rotations and fungicide use, as well as management aspects such as sowing between rows and stubble management, are all important for minimising yield losses.
An 18 to 24-month break from host crops and grass weeds, by rotating with canola or pulses for example, is an important control strategy.
Seed fungicide treatments are becoming more helpful.
While varieties with the best resistance rating can still be severely impacted, they are better choices than susceptible varieties.
Genetic resistance is very good for some wheat diseases such as the three rusts, stem, leaf and stripe, Septoria tritici and yellow leaf spot.
Unfortunately, it is a major breeding challenge to develop varieties with high resistance ratings to all of these diseases.
And to make things more difficult, diseases can mutate and develop tolerance to genes providing resistance.
Hence breeders aim to develop varieties with several resistance genes to a specific disease, which generally helps for longer resistance.
Most areas can be impacted by a number of diseases.
Choosing varieties with the best resistance to the biggest risk, for example stripe rust, is a priority.
Checking resistance rating of chosen varieties to other likely diseases and having other control strategies, is also important.
For example, in a wet late winter early spring, Septoria and yellow leaf spot, and leaf and stem rust, can also develop into serious problems.
Chickpeas are an example of fungicides combined with genetic resistance, as well as rotations and other agronomy aspects like disease free seed, as all being important for control of troublesome diseases like Ascochyta and phytophthora root rot.
An MS rating (moderately susceptible) is the best present Ascochyta resistance available and for maximum disease protection in a wetter year needs to combine with a sound fungicide program.
Fungicide programs include treated seed plus foliar applications.
Disease threats vary from disease to disease and crop type.
Blackleg in canola is especially unique.
New strains of the disease are a constant and choosing hybrids or varieties with specific resistance is important.
Agronomy management, such as avoiding disease carryover on stubble from one and even two years previously, seed fungicide treatment and foliar fungicide control are all important control options.
There are many other aspects requiring assessment when choosing varieties of all crops.
Yield rating is important, as are all disease threats, quality aspects and sowing time suitability.
Two nematodes, P. thornei and P. neglectus, can severely affect yield in several crops including wheat barley and chickpeas.
Variety has resistance and tolerance is variable, some poor and vulnerability to big yield losses should pests numbers be high.
Rotations with resistant crops is a sound control strategy.
A good guide to helping choose the most suitable varieties for various risk situations is the NSW DPI Winter Crop Variety Sowing Guide, 2023.
While new varieties will be released for 2024, the vast amount of crop varieties likely to be sown next year are well detailed in the publication.
Next week: Sulphur vital for canola as well as pasture production.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
