Background and aim
Legume break crops are an important part of the farming system. In the northern agricultural region, chickpea production reached around 80 000 tonnes in 1999, before the occurrence of ascochyta blight. Chickpea is well adapted to the northern region and as new varieties are released there in a need to ensure they are managed effectively. The aim of this trial was to develop variety specific packages for managing ascochyta blight.
Plots of three chickpea varieties Howzat, Genesis 836 and PBA Striker with varying tolerance to the foliar disease of chickpeas Ascochyta rabiei; were sown on 2 May at Yandanooka (South Mingenew). Plots were 10 metres long by 2.0 metres wide. Buffer plots of lupin were sown between each chickpea plot to stop the spread of disease between treatments.
Chickpea plots were inoculated with chickpea stubble (sourced from an ascochyta disease nursery at Medina) which had obvious ascochyta lesions. Three 10cm long sticks of diseased stubble were placed in the ground 2m in from each end of the plot. Half of the plots were inoculated early on 2 June. This introduction of inoculum simulated an early occurrence of disease that may occur due to using infected seed or placing a chickpea crop too close to a crop from the previous session. A further introduction of ascochyta infected stubble was made to the remaining half of the plots on 5 August, to simulate a late disease challenge.
Each of the resulting six treatments had a different foliar fungicide treatment; nil (none), one application early in the season (2 June) one application late in the season (11 August), two applications (2 June and 11 August) and fortnightly application (control). The fungicide used was 1.0L/ha Barrack® (chlorothalonil 720g/L) applied in 100L water/ha.
Three treatments were used;
- Three chickpea varieties (PBA Striker, Genesis836 and Howat)
- Two times of ascochyta introduction to the trial (early - 2 June 2, late - 5 August)
- Five fungicide spray regimes, as described above.
The trial was sown into good moisture at on2 May and rainfall through the growing season totalled 283mm, close to the long term average May-October rainfall of 310mm. There were 11 rain days in June (all after the first inoculation of ascochyta infected stubble); six in July, two in early August and a further 10 after the late introduction of disease on 5 August. Hence it was a season of frequent rainfall events which would be expected to stimulate ascochyta infection.
*Growing season rainfall
Plant density was lower than the 40-45 plants/m2 target at 32 plants/m2 because good follow up rain did not occur until 22 May. Despite this there was no statistical difference between the plant density of the varieties; Genesis 836, Howzat and PBA Striker at 33, 33 and 32 plants/m2.
NDVI measurements were taken at four dates throughout the season: (23 June, 14 July, 28 July and 5 August). At the earlier times of measurement the disease inoculation and fungicide spray regimes had not yet resulted in differences in green leaf area between plots and this was confirmed by the NDVI measurements. However by 5 August, there was a significant reduction in NDVI readings which were related to the varying amounts of disease and necrosis within plots. There was more green area in the moderately tolerant variety PBA Striker plots compared to the other varieties (P <0.001) with the susceptible variety Howzat having the lowest green area. Plots in which infection had been introduced earlier had less green area than later infected plots. The plots sprayed with fungicides fortnightly had more green area than all the other fungicide regimes (P <0.001).
Dry matter cuts taken at maturity, 24 October, indicated that variety, spray regime and inoculation time all had significant effects on plant growth (Figure 1). This followed the same pattern as NDVI and ascochyta ratings with Howzat having reduced biomass compared to the other varieties. There was less biomass from early infected plots compared to late infected plots and the fortnightly control treatment had more biomass than other fungicide treatments, whilst the nil fungicide plots produced the least biomass. None of the interactions were statistically significant.
Ratings for extent and severity of ascochyta disease were taken on 5 August, the same day as the late infection plots had the diseased stubble placed in them. These ratings showed that the early infection and different spray regimes applied up-to that date had resulted in significant differences in the amount of ascochyta within the plots. Howzat plots had more ascochyta in them than PBA Striker and Genesis 836 (P <0.001), and there was more disease in early infected plots compered to late infected plots (P <0.001), the nil fungicide treatment had the most disease infection and the control fortnightly spray the least (P <0.001). The interactions Variety*inoculation time, Variety*spray regime, Inoculation time*spray regime and variety*inoculation time*spray regime were all highly significant. A rating for ascochyta taken towards then end of the growing period on 24 October showed the same responses.
Yields were quite low considering the favourable season, site mean was 700kg/ha although this was reduced heavily by the disease that was introduced within the treatments. The control treatments averaged across both disease inoculation dates yielded 800, 754 and 878kg/ha for Howzat, Genesis 836 and PBA Striker respectively. Yield was impacted by the treatments in the same way as NDVI, biomass and ascochyta. Howzat, the variety with least resistance to ascochyta suffered large yield losses with early disease introduction; in fact it was difficult to ensure control plots remained disease free even with fortnightly fungicide application. For the more resistant varieties, Genesis 836 and PBA Striker yield declined when fungicides were not applied. When disease was introduced later the treatments responses were less consistent. Yield of Howzat still declined when it had no fungicides applied and the yield of Genesis 836 and PBA Striker declined under some spray regimes but the nils of these varieties yield well showing their genetic tolerance to the disease. As would be expected the relationships between ascochyta disease to growth and yield were stronger when the level of disease was high.
Seed weight was affected by variety, with Genesis 836 producing the lightest seed (18.3g/100 seed) followed by Howzat (21.6g/100 seed and PBA Striker (22.8g/100 seed). Spray regime also affected seed weight with control seed heaviest and nil fungicide treatment seed the lightest. Seed from early ascochyta infection was lighter than the later infection - but this was not significant.
The trial demonstrates that new chickpea varieties have a greater tolerance to ascochyta than older varieties; nevertheless diligent management of ascochyta to stop early infection is critical. This includes ensuring seed is disease free and treated with fungicide to stop early infection and that chickpeas are not sown close to last years chickpea stubble.
Both Genesis 836 and PBA Striker showed greater tolerance of the disease than Howzat however a two spray fungicide spray regime is still recommended to ensure yield is maintained and disease is kept at levels that will not build up in future crops. This should be more achievable now compared to 15 years ago when ascochyta first occurred because fungicides are now routinely applied to other crops and as such farmers have increased application capacity.
Thanks to Stephanie Boyce and the DAFWA Geraldton RSU for trial management and measurements and David Bagley and the Mingenew Irwin Group for supplying the site. This trial (16GE04) was funded by Department of Agriculture and Food, Western Australia and Grains Research and Development Corporation (DAW00227).