Pulse diseases the watch outs for 2016
Author: Kurt Lindbeck (NSW Department of Primary Industries) | Date: 16 Feb 2016
The growing season was challenging for pulse crops in 2015. In most districts lupin and faba bean crops were sown on time following good rains in April. Drier conditions in May and early June slowed crop development but also kept early foliar disease development low. Cool and wet conditions in late June, July and August provided much needed soil moisture, but were also ideal for foliar disease development. Long periods of leaf wetness provided plenty of opportunities for pathogens to infect and become established. Drier than average spring conditions halted further foliar disease development across the region but this lead to many pulse crops becoming stressed for moisture. High temperatures during flowering and pod filling also kept yield potentials low in many crops.
Brown leaf spot appeared in many crops across southern NSW during wet conditions in winter. The disease survives as soil borne spores that are splashed onto crop foliage during rainfall events and produce a distinct brown leaf lesion. The fungus colonises the leaf and the leaf dies prematurely. The disease is most damaging during the early stages of crop establishment when plants are small. Early infection can lead to the complete defoliation and premature death of plants. Management of the disease includes crop rotation (at least a five year gap between lupin crops to allow spore numbers to decline in soil). Avoid sowing this year’s lupin crop adjacent to last year’s paddock. Also avoid use of a fungicide seed dressing and avoid sowing into cereal stubble to avoid rain splash of spores. There are no foliar fungicides currently registered for use to manage brown leaf spot.
Phomposis stem blight was detected in late October in New South Wales. A combination of wet winter conditions, which provided excellent conditions for infection by the Phomopsis fungus, and moisture stress in spring was ideal for early development and expression of the disease. Interestingly the disease was observed mainly in albus lupin crops, in particular the variety Rosetta, but samples were also received at Wagga Wagga of the disease in other albus lupin varieties. A general warning about lupinosis was circulated in 2015 following the detection of the disease in commercial crops and the risk to livestock. Due to the sporadic nature of disease outbreaks, management of the disease relies on separating this year’s lupin crop away from last year’s lupin stubble, as the pathogen survives on old lupin stubbles and releases air borne spores in winter. Some varieties have better levels of resistance than others.
Phytophthora root rot (also known as Sudden Death) developed in some crops in mid spring. This disease develops following a period of waterlogging in winter and is expressed in spring when plants prematurely die, often as the soil starts to dry out and the infected root system can no longer support the plant. Soils prone to waterlogging, low lying paddocks and hardpans (which can develop perched water tables) can all contribute to the development of this disease.
The main diseases of faba bean in southern NSW and northern Victoria are ascochyta blight (caused by Ascochyta fabae), chocolate spot (caused by Botrytis fabae and Botrytis cinerea) and rust (caused by Uromyces viciae-fabae). These fungal pathogens have the potential to reduce crop yield and seed quality. Disease management strategies for these diseases have been developed that utilise a range of chemical and non-chemical approaches, such as paddock selection, crop rotation, selection of seed for sowing, variety selection, sowing date and rate, and the strategic use of foliar fungicides. At this point in time producers still rely heavily on fungicides as part of a disease management strategy and success is dependent on correct disease identification, timing of product application and fungicide choice. Wet conditions in winter 2015 were ideal for the establishment of foliar diseases in faba bean and made fungicide applications challenging. Drier conditions in spring slowed down the spread of disease.
Ascochyta blight develops early in the growing season, especially when conditions are cool and wet for prolonged periods. Early infection results in the development of leaf lesions. These later spread and develop into stem lesions in susceptible varieties. This can cause premature lodging in susceptible varieties. Later in the season, after the commencement of flowering, the disease can spread onto developing pods and cause seed staining.
Chocolate spot is favoured by warm, humid conditions for prolonged periods (four-five days). Typically the disease develops later in the growing season as crops commence flowering and after canopy closure. Canopy closure is an important crop growth stage and marks the point where the crop’s canopy develops from single rows within the paddock to a continual cover of canopy, which then creates a humid microclimate. Chocolate spot has the potential to develop quickly when conditions are favourable and results in pod abortion and plant damage (through leaf infection and loss of leaf tissue).
Rust can be found from mid spring and is favoured by warm temperatures (above 20°C). The disease can develop very quickly, requiring only six hours of leaf wetness for infection. Rust is not usually a problem every year in southern regions, and often occurs in years with good spring rainfall and mild temperatures.
Disease management strategy – the critical periods
There are three critical periods for monitoring faba bean crops for disease:
- 1st Critical Period is five - eight weeks following emergence. Ascochyta blight is the main target at this time and the use of foliar fungicides is aimed at reducing early establishment of the disease in crop
- 2nd Critical Period is during early flowering just prior to canopy closure. This is the last opportunity to apply a fungicide that will penetrate into the crop canopy and protect potential infection sites from disease establishment and spread. Chocolate spot is the target disease at this time and the prevention of this disease becoming established under the crop canopy. In ascochyta blight susceptible varieties, a fungicide application at this time will help prevent further spread of this disease
- 3rd Critical Period is at the end of flowering and early pod fill. Applications of fungicide at this time should be aimed at protecting developing pods and preventing any further disease spread. The target diseases at this time are ascochyta blight, chocolate spot and rust.
There are a number of registered foliar fungicides for use on faba bean to manage ascochyta, chocolate spot and rust. Some products are broad spectrum and are effective against a number of diseases (such as products containing mancozeb and chlorothalonil), while others are specific against single diseases (such as products containing carbendazim and procymidone).
Correct disease identification is important as this will determine the choice of product. Currently registered products are listed in the NSW DPI Winter crop variety sowing guide.
Blackspot (also known as Ascochyta blight of field pea) is the most common foliar disease of field pea. The disease is caused by a complex of four fungi, which between them can survive on seed, stubble and in soil. Development of the disease is favoured by cool, wet conditions. Symptoms of the disease include distinct round, black lesions which form on leaves, elongated round lesions which form on stems, blackening of the stem at ground level and pod lesions. Early epidemics of the disease can result in significant yield loss, but most crops in southern NSW develop the disease later in the season and suffer very little, if any, yield loss. Management options include crop rotation, paddock selection, sowing time (avoidance of ascospore showers early in the season from old stubble), fungicide seed dressing and foliar fungicides.
Bacterial blight was not widespread in 2015, with few reports of the disease in southern NSW. Reports of bacterial blight outbreaks were received from central and northern NSW where frost events in July and August caused damage to more advanced field pea crops compared to the southern districts. Slower developing crops and fewer frost events in southern NSW did not cause crop injury, and hence, outbreaks of the disease.
Hardpans, waterlogging and nodulation
Reports were received during 2015 of poor performance in many pulse crops across southern NSW, regardless of crop species. Samples received at the diagnostic laboratory at Wagga Wagga indicated a number of non-disease issues in pulse crops across the region.
A number of lupin and faba bean (and canola) samples received had classic ‘L’ root symptoms due to hardpans. Hardpans restrict taproot growth to the depth of the sowing furrow. This also means that moisture lower in the soil profile cannot be accessed later in the season and can cause crops to reach senescence prematurely. Surveys in the mid-1990s found over 50 per cent of paddock in southern NSW to have a hardpan problem (the bulk density was high enough to restrict taproot growth).
Hardpans can also lead to waterlogging issues. Sudden downpours or long periods of heavy rain can result in the development of perched water tables in the root zone. This can result in root disease development, poor nodulation or poor root growth. Paddocks prone to waterlogging (terrain) or heavy soil types can have the same affect.
A number of samples were also received with very poor nodulation. This may have been due to a number of factors including:
- poor application of rhizobia at sowing
- soil pH
- herbicide residues.
Poor nodulation results in poor nitrogen fixation and poor crop performance. To maximise the benefits of a pulse crop in the rotation, maximising rhizobium survival and colonisation of the root system is essential. Consideration has to be given to previous herbicide use, soil pH, soil type and drainage and how this may impact on rhizobium survival and pulse crop performance.
NSW DPI Winter crop variety sowing guide (Disease updates, variety resistance, fungicide products)
Funding for this work was provided through the GRDC Project DAN177 and their support gratefully acknowledged.
NSW Department of Primary Industries,
Wagga Wagga Agricultural Institute
02 6938 1608
GRDC Project code: DAN177
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