Changes in Ascochyta blight in pulse crops

Chickpeas

Ascochyta blight was observed in chickpea crops in SA including Genesis™090. Severity in Genesis™090 was low to moderate in SA but severe infection on this variety was observed in at least one site in Victoria. Isolates from these two states were tested at SARDI in controlled conditions against an old SA isolate. Reactions shows that Genesis™090 has a susceptible reaction to the Victorian isolate but not to the other two isolates. Ambar, Neelam and Genesis™079 demonstrated resistance to all three isolates; Almaz had a moderate reaction to the new SA isolate but a good level of resistance to the other two isolates; PBA Maiden had a moderate reaction to all three isolates; Genesis Kalkee, PBA Monarch, and PBA Slasher have a moderate to susceptible reactions to all three isolates while PBA Striker had a susceptible reaction to all three isolates (Figure 1). These results still need to be confirmed with field data.

Figure 1: Reaction of chickpea NVT lines against three isolates of A. rabiei tested in controlled conditions at SARDI, December 2015.

Lentils and faba beans

Changes in Ascochyta blight reactions on cultivars are being monitored on faba beans and lentils. Collections of isolates have been made from SA, Victoria and New South Wales and these have been tested against specific cultivars in controlled conditions to identify any changes in response.  Research of previous years identified new pathotypes in both A. lentis and A. fabae in major growing regions, that is, resistance in Nipper lentils is no longer effective in SA nor Victoria, and resistance in Farah faba beans and partial resistance in PBA Rana have been overcome in Mid North of SA.

Sixty three isolates of A. lentis collected during 2014 were tested in controlled conditions on five hosts including Nipper and Northfield. Seventy per cent were aggressive on Nipper and 90 per cent were aggressive on Northfield, an increase over previous years in which less than 20 per cent of isolates could infect one or other of these cultivars. These cultivars need to be managed similar to Nugget with a fungicide spray during podding to prevent pod infection and abortion, and in high risk situations may require a spray during early flowering. This study has found that the population of A. lentis is naturally variable and high intensity cropping of a single variety leads to the selection of the resistance breaking isolates. Consequently, the rapid and dominant uptake of PBA Hurricane XT, particularly on Yorke Peninsula, threatens the longevity of Ascochyta blight resistance in this cultivar. In 2015 Ascochyta blight was observed in susceptible lentil crops such as PBA Flash, and very small lesions of this disease were also detected in a few PBA Hurricane XT crops. If resistance in PBA Hurricane XT becomes ineffective this will potentially impact on numerous other cultivars and breeding lines that share a common resistance. 

Thirty eight isolates of A. fabae collected during 2014 and 2015 were tested in controlled conditions on five hosts including PBA Rana, Nura and PBA Samira. The majority of the isolates (71%) were the new pathotype that is virulent on PBA Rana and Farah. The majority of these were collected from crops and trials in the Mid North region of SA, confirming this as the major region affected by the change, although two isolates were from Victoria and a weakly aggressive isolate was from the south east of SA, indicating a potential for further spread of the new pathotype. A test of 25 historical isolates collected through the Mid North from 1991 to 2013 were also tested and showed that the new pathotype was present in 2012, one year earlier than field observations detected the change. Faba bean crops should be monitored regularly for Ascochyta blight to allow timely fungicide responses and any anomalies in resistant cultivars reported to SARDI. 

Foliar fungicides for Ascochyta blight of field peas

Weather patterns experienced early in 2015 resulted in growers in many districts being advised by DAFWA’s Blackspot Manager Prediction model to sow field peas late. This timing was often out of alignment with optimal sowing times based on best agronomic practice for some districts. Growers in these districts had to decide whether to choose an alternative crop, sow field pea into high blackspot-risk situations, or delay sowing date past the optimal window for successful production. In situations such as these, growers could revise their blackspot management strategy and consider fungicide applications being recommended to manage this disease. This is becoming more feasible while grain prices are high. If going against the Blackspot Manager recommendations, and choosing to sow into periods where a high risk of blackspot spore showers are predicted in your region, growers should consider an alternative break crop to field pea. However, if field peas are preferred it is important to consider the following to reduce the risk of blackspot outbreaks:

• Apply P-pickel T® seed treatment (PPT) to seed prior to sowing and follow up with current recommended fungicide strategies of two applications of Mancozeb®, one at eight weeks after sowing and one early flowering ɸ
• Select paddocks with no history of field pea, or paddocks with a long break period from field pea and history of a low incidence of blackspot
• Avoid close proximity to previous field pea stubbles, particularly downstream to prevailing wind direction
• Delay sowing as long as possible.

ɸ Timing of spray application listed on the FMC and Innova mancozeb labels is: first sign of infection and repeat at seven to 10 day intervals (dry food consumption) or first sign of infection and repeat at two to three week intervals (fresh food consumption).

Other pulse disease observations in SA during 2015

PBA Hurricane XT is moderately susceptible to botrytis grey mould (BGM) and required one or two sprays before the dry spring set in while PBA Jumbo 2 exhibited high resistance to BGM. Frost conditions seemed to interact with BGM causing yellowing in a number of crops, especially on northern Yorke Peninsula.

Low levels of powdery mildew in field peas and rust on faba beans were observed in upper north crops, but the dry spring held these diseases back in most areas.

Field mould studies on faba beans

The agronomic practices of crop topping and windrowing on faba bean grain quality, as well as the impact of mechanical damage (e.g. wheel-tracks), were examined in field trials conducted in three growing regions (Eyre Peninsula, mid north and south east). This research is funded by SAGIT to examine the issue of field mould in faba bean, and is conducted in collaboration with the Southern Pulse Agronomy project funded by the GRDC. Several grain quality attributes were examined on seed harvested from the different treatments: total number of blemished seed (accumulative measure), 200 grain weight, grain uniformity (visual – colour and size score), number of weather-stained seed, number or shrivelled seed, number of seed with Ascochyta staining, number of seed with residual adhesions from pod wall. These parameters were used to measure seed quality, and their potential interaction with the incidence of field mould which has led to down grading or rejection lots at grain receival silos in past seasons.

The results from trials conducted in 2014 are discussed in this paper. That season was not favourable to the development of field mould on seed at the sites due to the mild and dry spring conditions. Though conductive conditions to disease were experienced in winter, the lack of spring rains did not perpetuate the development of Ascochyta staining on seed and minimised abiotic stresses affecting the developing pods. However, affects observed from this study can be used to provide a guide as to the dynamics of factors affecting seed development that may occur in seasons of wet conditions in spring or early summer.

• Variety has a significant effect on seed quality. The older varieties Fiesta and Nura (and Farah included in the EP trial) were consistently shown to have poorer seed quality than newer varieties PBA Rana (and PBA Zahra in the Mid North trials – see Table 1)
• Traffic damage (wheel-tracks) significantly affected seed quality at all three sites (Tables 2, 3 and 4). This effect was significantly exacerbated in Fiesta and Nura compared to PBA Rana seen by varietal differences at all sites. However, ‘flattened plants’ were actively ‘picked-up’ during harvest to examine seed, which may not occur in large scale commercial applications
• The practice of windrowing in itself had no adverse effect on seed quality (data not shown). Significant differences such as colour, grain weight and uniformity was driven by variety type (PBA Rana being superior to Fiesta and Nura)
• The influence of windrowing timing (early, late or nil), examined in the Mid North trial, had little effect on seed quality. However, when windrowed treatments were combined with a late harvest treatment (mimicking leaving windrow for longer period before pickup) a significant reduction in seed quality was seen (data not presented). Varieties Fiesta and Nura were more susceptible to this effect than PBA Rana
• In 2014, crop topping timing had a little to no effect on seed quality at all sites. However, mild and dry ripening conditions may have suppressed this effect compared to wetter conditions experienced in 2013, where an effect of crop topping on seed quality was shown by a trial in the Mid North
• The superior seed quality of PBA Rana showed it was more tolerant to the variation of agronomic practises that might affect seed quality examined in this study, compared to Fiesta and Nura which have less consistent colour and size.

Table 1: Crop topping x variety trial conducted at Tarlee, Mid North, in 2014 to examine effects on faba bean seed quality.