Brown spot (Pleiochaeta setosa) is the most widespread foliar disease of lupins in Western Australia. Paddocks which have previously grown lupins will have Pleiochaeta spores in the soil. The disease can infect lupins at all stages of growth but seedling infection has the greatest impact on yield. Spores produced on dead tissue become incorporated into the surface layers of the soil where they can persist for several years, although under non-host crops the concentration reduces over time. Infection occurs when spores are splashed by rain from the soil onto new lupin plants. Factors which reduce the growth rate of plants such as colder environments, late sowing, poor nutrition, herbicide damage or unfavourable soil type, prolong exposure to rain-splash at the most susceptible seedling stage. All lupin species are affected although yellow lupins show resistance.
Infected cotyledons develop dark brown spots and rapidly become yellow and drop off. Leaves also develop dark brown spots, often net-like in appearance and can be distorted and reduced in size before prematurely dropping off. On stems, brown flecks may be evident, occasionally developing into large brown-black cankers which kill the stem above the infection point. Pods, particularly those set closer to the ground, may be flecked or develop larger brown lesions. Stem and pod infection are usually associated with leaf infection in the upper canopy.
- Soil borne spore concentration diminishes under non-host crops, consequently longer rotations reduce risk of brown spot infection.
- Sowing lupin into retained cereal stubble reduces rain splash of soil-borne spores onto foliage.
- Application of iprodione or procymidone based seed dressing fungicides dramatically reduces seedling brown spot infection.
- Factors which promote seedling vigour and canopy closure such as early sowing, adequate nutrition, care in herbicide use, higher seeding rates and sowing onto favourable soils reduce disease impact.
- Small differences in brown spot tolerance exist between narrow-leafed lupin varieties but this does not remove requirement for other management approaches.