HALO BLIGHT IN MUNGBEAN - PATHOGEN, HOSTS, INFECTION, CROP IMPACT, MANAGEMENT AND FUTURE GENETICS AND BREEDING OPTIONS

 

Three major bacterial diseases of mungbean have been identified in Australia, halo blight (caused by Pseudomonas savastanoi pv. phaseolicola (P.s.phaseolicola), tan spot (caused by Curtobacterium flaccumfaciens pv. flaccumfaciens), and bacterial blight (caused by Xanthomonas axonodopis pv. phaseoli). Bacterial blight is uncommon, tan spot appears during warm weather and often when plants are stressed, while halo blight has become increasing more common and serious over the past 5 years.

 

The pathogen

Halo blight of mungbean was first recorded in Queensland in the early 1980’s. Overseas, there are at least 9 strains (races, pathotypes) of the bacterium, which has serious implications for resistance breeding – varieties that are immune to some strains may be susceptible to others.

 

Hosts 

Worldwide, P.s.phaseolicola is a major pathogen of French bean, and in Australia it has been recorded on that host as well as runner bean, lima bean, soybean, pigeon pea, siratro, and native glycines. Other major overseas hosts are lablab, field peas and Desmodium species.

 

Symptoms

Halo blight. Symptoms take 7-10 days to appear after infection. On young infected leaves there is an extensive yellow halo surrounding a small (1-2mm) dark, water-soaked (shiny) spot, while on older leaves the halo is not apparent. On infected pods a shiny circular lesion develops, and a cream-coloured drop containing millions of bacterial cells may ooze from the lesion.

 

Tan spot. Leaves infected with the tan spot pathogen develop large, irregular dead areas surrounded by a broad yellow margin, usually near the margins of leaves. Flowers and young pods can be killed by tan spot infection.

 

Survival and infection

P.s.phaseolicola, like the tan spot pathogen, can survive between growing seasons on alternative hosts, on and in infected seeds, and to a limited extent on infected plant residues. Overseas, infected seed is considered to be the major mode of survival and spread of the pathogen, and work with French bean has shown that only 1 infected seed in 10,000 is needed to start an outbreak, assuming the weather conditions are right. P.s.phaseolicola is spread during wet, windy weather, with infection occurring through natural openings (leaf stomates) or wounds when there is free moisture (from dew, rainfall or irrigation). Temperatures in the range 18-25ºC are most conducive for infection and disease development, meaning that spring-planted crops are at the greatest risk. There is no evidence that the halo blight pathogen moves inside the plant from infected leaves into the pods.

 

Crop impact

Early infection by P.s.phaseolicola causes plant stunting, which together with its impact on the photosynthetic capacity of leaves, can seriously impact on yields. Overseas, P.s.phaseolicola has been recorded to cause up to 40% yield losses in French bean crops. Yield losses are difficult to quantify because the disease is very hard to control through the use of copper-based sprays, but some preliminary work at Kingaroy suggests that yield losses in the order of 70% are possible.

 

Management

The most effective practices for the management of halo blight in mungbean are the use of clean seed, incorporation of infected plant residues, and plant resistance. Other options, such as, crop rotations with non-host crops, seed treatments and the spraying of infected crops with copper-based pesticides are likely to have a minimal impact on disease levels compared to the effects of the other strategies.

 

Resistance

Current commercial varieties have little resistance to the halo blight pathogen, being in the very susceptible to moderately susceptible range. However, through the combined efforts of the DPI&F mungbean breeding and pulse pathology teams several small-seeded and large-seeded mungbean lines with moderate resistance or resistance to P.s.phaseolicola have been identified. Figure 1 displays the relative levels of resistance of some of these breeding lines and commercial varieties to the halo blight pathogen. The lines with a rating of 1 are totally immune, suggesting that they may possess one or more major genes for resistance.

 

The upside of major genes is that breeding the resistance into new varieties can be relatively easy; the downside is that new strains of the pathogen may appear if such new varieties are released. We have just started to monitor the population of P.s.phaseolicola in the GRDC northern region to determine if such strains already exist.

 

 

Figure 1: Resistances of some mungbean varieties and breeding lines to P.s.phaseolicola

 

 

 

The 2 keys in managing halo blight

·         Clean seed – only use seed from a crop which has been officially inspected for the presence of halo blight and other pathogens

·         Resistance – use the most halo blight resistant varieties available

 

Contact details

Dr Malcolm Ryley

Principal Plant Pathologist

Department of Primary Industries and Fisheries, Toowoomba

Ph: (07) 4688 1316

Fx: (07) 4688 1199

Email malcolm.ryley@dpi.qld.gov.au