The fungus Pleiochaeta setosa causes both brown leaf spot and Pleiochaeta root rot of lupins. Brown leaf spot is considered to be the most widespread foliar lupin disease in Australia. Severe outbreaks of brown leaf spot can cause total crop failure, but more often the disease reduces potential yields by 10-15 percent.
Implementing management strategies is essential to protect lupin crops as there are no treatments available post sowing for Pleiochaeta setosa.
Pleiochaeta setosa can cause both a root rot and foliage infection and can infect lupins at all stages of growth though seedling infection has the greatest impact on yield.
In the paddock wilted, weak or dying seedlings can be scattered throughout and will reduce stand density, plant vigour and yield. Incidence may be worse in paddocks with close lupin rotations, though any paddock that has previously grown lupins will have pleiochaeta spores in the soil.
Plants will germinate normally, severely affected seedlings will wilt and die displaying dark-brown or black lesions on the taproot that causes the root to rot, (Figure 9.1). Less affected seedlings are weaker and smaller than unaffected plants and root lesions can strip off the outer layer of tap roots that prevents nodulation. After the 6 to 8 leaf stage the taproot thickens becoming less susceptible to infection. However the lateral roots are susceptible for longer and often an inspection will reveal a lack of lateral roots.
Figure 9.1 Typical root rot symptoms caused by Pleiochaeta setosa. Note; girdling of taproot by lesion.
Infected cotyledons develop dark brown spots and rapidly become yellow and drop off, (Figure 9.2). Leaves also develop dark brown spots, net-like in appearance and can be distorted and reduced in size and under severe infection will drop off causing partial to complete defoliation, (Figure 9.3). On stems, brown flecks may appear which can develop into large brown-black cankers which kill the stem above the infections point. Pods, especially those closer to the ground may be flecked and develop larger brown lesions. Stem and pod infections are usually associated with leaf infection in the upper canopy.
Figure 9.2 Typical brown leaf spot leaf lesions. These irregular shaped, brown spots are caused by Pleiochaeta setosa infection.
Figure 9.3 A badly defoliated lupin seedling. Leaves have dropped off due to severe infection by Pleiochaeta setosa.
During the growing season large numbers of spores are produced when diseased leaves fall onto the soil surface. These spores start new infections through rain splashed foliage.
The pathogen is carried over from one season to the next on previously infected plant material, in infested seed or as spores on the soil surface. Cultivation and sowing incorporate spores into the surface layer of soil where they remain dormant over summer.
Spores survive through non-lupin crops though their populations decline over time. When the next lupin crop is sown, soil-borne spores germinate and infect the roots of lupin seedlings, causing Pleiochaeta root rot. Spores that have survived on the soil surface are splashed upwards by rain droplets, infecting leaves and stems and causing brown leaf spot; thus continuing the disease cycle.
Seed-borne infections are important for dissemination of the pathogen over long distances and are responsible for initial infection in clean paddocks that are isolated from other lupin crops. Severely affected pods can contaminate seed lots and act as a source of infection after sowing. Once infection is established within the crop, secondary infection of other plant parts can occur by splash dispersal of fungal spores during rain, (Figure 9.4).
Figure 9.4 Infection cycle of the fungus Pleiochaeta setosa. Illustration by Kylie Fowler
Brown leaf spot and root rot can be effectively controlled when an integrated approach to disease management is implemented. This involves using a number of strategies including crop rotations, seed dressings, resistant cultivars and retaining cereal stubble.
Crop rotation is an important management strategy as the number of pleiochaeta spores in the soil is reduced by half for every year that a non-lupin crop or pasture is grown in the rotation. Reduced or minimum tillages operations reduce the incorporation of spores into the rooting zone of the soil profile. Long rotations are important so that lupin stubble will be decomposed before the next lupin crop is sown; avoid planting lupins in paddocks adjacent to lupin stubble.
Treat all seed with a recommended fungicide and ensure that seed lots are free from plant debris.
Deeper sowing places the emerging roots of lupins below the spores reducing the chances of pleiochaeta root rot. Sowing lupins into cereal stubble will reduce rain splash of spores onto lupin plants. The only other known host for brown leaf spot is serradella, a low- yielding legume which is not common in Victorian lupin areas.
Another important method for controlling Pleiochaeta setosa is to apply a fungicide seed dressing (containing ipodione or procymidone), although this only suppresses the disease and does not provide complete control but will reduce the transfer of the disease to the seedling and can reduce leaf drop by 50 percent.
Use resistant cultivars, (see Table 9.1). Variety selection is also an important management strategy. New narrow leaf lupin varieties (Lupinus angustifolius) have been released with resistance to pleiochaeta root rot and brown leaf spot. Broad leaf lupin (Lupinus albus) varieties are available with tolerance to brown leaf spot, but can be susceptible to root rots under wet conditions and so are limited to well-drained soils.
DISEASES IN LEGUMES: LUPINS
PHOMOPSIS STEM AND POD BLIGHT OF LUPINS