By Eric Armstrong, NSW Agriculture, and Trevor Bretag, Department of Primary Industries, Victoria
Bacterial blight has been widespread in field pea crops across southern NSW and the Wimmera region of Victoria this season, highlighting the need for growers to better understand the problem and the management options needed to reduce risk.
Though widespread, the actual extent of the damage varied with total crop failures occurring only in a few instances. These were largely lowlying crops that suffered early infection (in late winter), followed by showery, windy weather.
In most cases, infection occurred late in the season (early spring) and little or no production losses occurred. The disease was found in most pea varieties, although Excell and Kaspa appeared to be the most seriously affected.
The most likely cause for blight outbreaks is physical damage to plants, such as the impact of frost, which provides an entry point for the bacteria.
The bacteria exist at low levels in most crops. However the disease is seldom a problem without some form of crop damage.
While pea plants are tolerant to frosts in the vegetative stage, flowers and developing seeds can be sensitive. Flowers opening when frost occurs are often aborted, leaving barren stalks at the affected node.
Developing seeds can be ‘snap frozen’ in the pod and killed. Depending on the stage of pod development, this will result in either small black seed remnants or shrivelled seed.
There are two pathovars of bacterial blight – Pseudomonas syringae pv pisi and Pseudomonas syringae pv syringae. In the past, pisi has been considered the most damaging pathovar in Australia and the current seed test has been designed to detect this strain.
However in recent years, particularly virulent forms of syringae have been detected in southern NSW and Victoria, and the current commercial seed test will only detect pisi.
Both strains are able to survive in seed and on infected plant residues for several years, although infection levels decline over time. Pea crop debris and volunteer plants maintain high levels of bacteria for at least one year after the harvest of a diseased crop.
The disease is usually introduced into new pea growing areas on infected seed. Weeds are also able to harbour both pathogens and may be important in the disease’s survival and spread.
If possible, only retain seed from crops that are known to be free of bacterial blight. Be aware that many crops have very low (often undetectable) levels of bacterial blight. Select the cleanest part of the paddock and harvest this first for your own seed. Inspections to identify this area must occur by flowering or earlier and follow this through until grain-fill.
The disease can be spread by farm machinery and grain handling equipment. Infected leaves and other plant parts become a significant source of further contamination during harvest and when seed is augured and cleaned. For this reason, handle different seed crops in isolation, running other grains such as cereals between the different seed batches to flush the equipment.
For more information:
Trevor Bretag, 03 5362 2111, Department of Primary Industries, Victoria, email@example.com
Pulse Point 13 (2001) – Strategies to minimise bacterial blight in field peas, available from NSW Agriculture, http://www.agric.nsw.gov.au/reader/pulsepoints
Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.
By Eammon Conaghan
What about a nitrogen-fixing legume with broad soil adaptability and drought tolerance? What if, while awakening tired soils to drive subsequent cereal protein levels up by two percent, this crop itself delivered grain worth around $260 per tonne?
Now add herbicide tolerance to the mix, allowing growers to spray late in the season and slash weed seed set by more than 90 percent in problem paddocks.
Well, it is not some magic crop yet to emerge from an R&D pipeline – it is the good old field pea. The hiccup is, of course, the fungal disease, black spot, which cuts yield by 20 to 40 percent and has hampered the crop’s adoption across more than one million southern Australian hectares.
A common feeling among growers and researchers is that a dramatic genetic defence is needed to help the crop overcome the disease. However there is one GRDC-supported project that is taking a more low-ley approach.
University of WA PhD student Cameron Beeck, and supervisors Dr Janet Wroth, Associate Professor Wallace Cowling and Dr Tanveer Khan, are taking the approach that when battling black spot, minor weapons can produce major results.
“Combining minor resistance genes provides collective strength to fight the disease, so we’re marshalling an army of these genes to work together for black spot resistance,” Mr Beeck explains.
This ‘genetic sandbagging’ requires researchers to accumulate the different genetic sources of resistance and transfer them into a single plant.
"To do this, we’re cycling through several field pea generations to simultaneously pursue different sources of genetic resistance. The key objective is to bring them together into agronomically adapted lines,” Mr Beeck says.
The ‘rapid cycling’ technique is, effectively, a series of concurrent projects which channel into one final product. While it is challenging to manage, it provides tremendous time and cost savings.
Apart from concentrating numerous minor genetic sources of black spot resistance into one variety, the GRDC project is also endeavouring to develop stronger plant stems to improve harvestability and adaptation to farming systems in southern Australia.
Following the first ‘rapid cycle’, some progeny have better disease resistance and stem strength than their parents. The team expects even greater improvement after further cycles.
“Once candidates with increased black spot resistance and stem strength are developed, those qualities will be combined and the best lines tested in multi-environment trials prior to release to growers or breeders,” Mr Beeck says.
For more information:
Dr Janet Wroth, UWA, 08 9380 2553
GRDC Research Code: UWA 337, program 2