The area of chickpea affected by Ascochyta Blight, (caused by the fungus Phoma rabiei, formerly Ascochyta rabiei) has increased considerably since the last update on 14 August 2014. Ascochyta blight has been found north in the Darling Downs of Southern Queensland and south at Forbes in central NSW. To the east, infected crops were found at Yallaroi and west at Tullamore in central NSW.
Despite this widespread infection, in most cases the disease is not causing significant damage at this stage. Good management through early and timely intervention with fungicides means few to no crops will sustain yield loss.
Most of the Ascochyta has been found in the variety PBA HatTrick. This is not surprising as this is the most commonly grown variety in the GRDC northern region. Ascochyta has also been found in PBA Boundary and Flipper.
Dr Kevin Moore is still surveying chickpea crops in the northern region but since the last update, a further 27 crops have been found with Ascochyta, bringing the total this season to 54.
Why is Ascochyta so prevalent this season?
The Ascochyta pathogen survives between seasons on infected plant residues, on infected seeds and on infected volunteer chickpea plants. Fields with chickpea stubble should be considered as a source of inoculum even if the disease was not found in the previous crop (Figure 1).
Even though there was very little Ascochyta reported in the previous two chickpea seasons, the pathogen is able to survive in both stubble and on chickpea volunteers thereby providing inoculum for this season. In addition, inoculum can spread through infected stubble blown about during and after harvest.
The other reason we are finding so much Ascochyta in 2014 is the weather. Disease spread and infection can only occur during a rainfall event. Since mid-June to mid-September there were 17 infection events in the Dubbo region, 19 in Parkes and up to 15 in the Moree district.
Australian chickpea varieties differ in their susceptibility to Ascochyta. Whilst plant breeders have achieved great advances in improving Ascochyta resistance, no chickpea variety is immune. All varieties get infected to some extent, so all are hosts.
Thus the three components of the disease triangle i.e. presence of inoculum (spores), a suitable host (chickpea) and the right environmental conditions (rain) have been met and this explains the widespread occurrence of Ascochyta blight on chickpeas this year.
Ascochyta found in resistant varieties – what is happening?
Plants subjected to less than ideal growing conditions are considered to be under stress. Stress impacts on growth, survival and crop yields as well as the plant’s resistance to disease. An example of this was found in a trial at Forbes where the chickpea variety PBA Slasher was infected with Ascochyta, yet PBA Striker and PBA Boundary grown only a short distance away showed no sign of the disease.
This trial was surrounded by a wheat crop which had been treated with a hormone herbicide to control broadleaf weeds. Unfortunately herbicide had drifted over the entire trial site causing typical hormone damage in the broadleaf species in the trial. The occurrence of Ascochyta on a variety rated Resistant (R) to Ascochyta i.e. PBA Slasher and the absence of the disease on varieties rated Moderately Resistant (MR) to Ascochyta i.e. PBA Striker and PBA Boundary, may reflect the greater sensitivity of Slasher to the hormone herbicide, which may have compromised its resistance to Ascochyta.
Similar observations have been made where parts of a chickpea crop subjected to waterlogging had greater levels of Ascochyta than parts of the crop that were not waterlogged (Figure 2)
When plants are exposed to stress (herbicides, waterlogging, salinity, sodicity, drought and other environmental conditions) their resistance to disease is compromised and they become more vulnerable to infection.
Managing Ascochyta blight
All growers and advisors need to regularly inspect their chickpea crops, right up to plant maturity. Inspections should be undertaken 10-14 days after rain events, when new infections will be clearly evident. Where Ascochyta blight infection is suspected, growers are advised to have the disease positively identified. According to Dr Moore, once Ascochyta is confirmed, growers should consider applying a registered fungicide based on chlorothalonil or mancozeb as close as possible to the next rain event.
Visual effects that are NOT Ascochyta blight.
Frost is the most common cause of leaf necrosis (whitening, death) which might be confused with Ascochyta blight. Herbicide residues (eg: simazine, Balance®) and nutritional disorders may also cause similar effects. Branches can be killed by frost, and physical damage from wind, mice and other animals, as well as the passage of machinery.
However, in these cases pycnidia (the fruiting bodies of the Ascochyta fungus) will be absent from the affected plant parts.
For more information:
- Pulse Australia has a number of publications on Ascochyta blight in chickpea including symptoms, identification and management.
- Managing fungal diseases of chickpeas GRDC
If Ascochyta is suspected plant samples should be sent to:
- Kevin Moore, NSW DPI, 4 Marsden Park Rd, Calala NSW 2340
- Sue Thompson, University of Southern Queensland Centre for Crop Health, West Street, Toowoomba Qld 4350
Samples should be wrapped in newspaper or paper towel and placed into an express post envelope (the plastic ones); ideally sent on a Monday or Tuesday not Thursday or Friday as they may be rotten after sitting in the post over the weekend; if needs be, samples can be stored in a fridge over the weekend before sending.
The following information should be included with the sample – name, phone number and email address, location (including nearest town, property name) and variety.
The GRDC is funding an ascochyta research project (UM00052) which is conducted by the University of Melbourne. This project is looking at the variability of the ascochyta pathogen population – to determine whether the pathogen is changing in response to the resistance in varieties OR whether the pathogen is becoming more virulent. Thank you to all growers and advisors who have recently participated in the collection of isolates.