Puccinia allii — onion and leek rust (also classified according to host range: P. porri, P. mixta, P. blasdalei, Uromyces ambiguus and U. duris).
Significant damage is caused by rust to allium crops worldwide including onion, leek, garlic and chives. There are considerable physiological and morphological differences between isolates of P. allii found internationally, leading to differences in host range.
For example, leek rust is prevalent in Europe but not frequently seen in California, USA, whereas there epidemics are common on garlic. Considerable crop losses are reported caused by P. allii around the world in countries as diverse as Chile, Israel, Norway and Tanzania.
Many rusts have four separate reproductive spore stages divided between two taxonomically different hosts (macrocyclic types). Others may be restricted to growing and reproducing on one host. Isolates of P. allii appear capable of living solely on allium species. Importantly, this pathogen produces orange pustules filled with uredospores, which are a dispersive phase allowing movement between and within crops.
Occasionally darker spores (teliospores) may be formed. It is reported that the other stages of
P. allii have been found on chives and Welsh onion (A. fistulosum).
This disease is seen initially as small (1-2 mm diameter) leaf flecks or spots. Usually these are irregularly shaped and white to tan coloured. Initially signs of infection are seen solely as light-green spots on the leaves that eventually become diseased and contain reproductive structures (uredinia).
Lesions grow in size as they mature, becoming 3-5 mm pustules, bright-orange filled uredospores. Pustules are found on both leaf surfaces, erupting through the epidermis, releasing clouds of dusty orange uredospores. Normally the pustules are restricted by the leaf veins running in parallel along the surface. Yellowing halos may develop around each pustule as it matures.
Older leaves are usually attacked first, with disease spreading onto the younger foliage as the epidemic progresses. As severity increases the leaves become covered in pustules, turn yellow, wither and die. Infected plants lose productivity and are smaller than uninfected ones, causing yield loss. Rust infection stunts plants such as garlic and the bulb sheaths may split and shatter, reducing quality and acceptability.
In some instances invading uredospores land on leek or other alliums and fail to establish infection. This leads to cosmetic damage seen as leaf spotting. This causes little harm to the plant but may substantially reduce quality in terms of lost compliance with supermarket specifications and can cause consequential loss of income.
Overwintered allium crops are a reservoir of infection that moves on to new spring-sown or planted ones. Rust is most active in the UK in midsummer, from July onwards. Disease progress slows as temperatures drop in the autumn.
Infection is optimal at 15°C and 100 per cent relative humidity for four hours. The fungus is active at temperatures of 10-24°C. The primary inoculum of uredospores is spread by wind currents and may move over considerable distances by this means.
Leeks demand large applications of nitrogen for successful cropping but excessive applications are associated with rust epidemics. Formulations of potassium may be used as a means of reducing infection levels. As with many diseases, crops that are placed under stress from drought or excessive irrigation are prone to infection. Wet conditions are conducive to disease development.
Integrated disease management
Resistant cultivars Use resistant cultivars, but growers should be vigilant against regularly using cultivars with similar genetic resistance because this will result in the appearance of physiological races of P. allii, resulting in tolerance to those genes.
Husbandry measures Weed alliums are colonised by P. allii and hence should be removed manually, mechanically or using approved herbicides. All crop residues and weed alliums should be ploughed deeply into the soil, where they may be destroyed by microbial action.
Crop rotation with non-allium hosts such as legumes or brassicas helps reduce the chances of disease outbreaks. Land should not be cropped with successive allium crops, especially leeks in Europe or garlic in California, USA.
It is now possible to grow leeks virtually year-round, for which there is substantial market demand. Consequently, growers are tempted to produce crops continuously that become very vulnerable to this pathogen. Early-spring transplanted crops tend to avoid disease outbreaks and the vulnerability increases as the season advances.
Denser planting is associated with increasing incidence of rust disease, so that the use of wider spacing that allows increased air movement within the crop is advisable. Renting good class-one agricultural land for leek production is a means for largely avoiding leek rust.
Forms of biological control have been advocated especially for organic crops using preparations of Bacillus cereus.
Currently allowable chemical treatments include:
• Azoxystrobin + difenoconazole (qualified minor use).
• Boscalid + pyraclostrobin
• Dimethomrph + mancozeb (reduction).
• Tebuconazole + trifloxystrobin.
For use on all fresh produce, vegetable crop growers must in advance of use ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and is also accepted by the intended purchaser’s crop-quality standards specification as agreed with the relevant crop technologist.