Blackleg TH E WOR LD OF atroseptica and E. carotovorum and E. chrysanthemi bacteria that cause soft rot in potato stems and tubers: blackleg. These bacterial species are named after an American bacteriologist Erwin Frink Smith. The first two Erwinias are now called Pectobacterium. E. chrysanthemi is now considered a complete different species and is named Dickeya after another American bacteriologist who did a lot of research on Erwinias Robert S. Dickey. Professor Dickey worked at Cornell University and died in 1991. Until recently most blackleg infections associated with Dickeya were associated with D. dianthicola that multiplies well in potato tubers at relatively low temperatures. When the number of bacteria is high they cause the tuber to rot. Bacterial infection of potato in recent years received increased attention with the arrival – since 2000 - of a novel soft rot and blackleg causing bacterium: Dickeya solani. This organism thrives at a higher temperature than its relative which it is taking over rapidly as it is more aggressive. The shift is likely responsible for the greater number of seed lots degraded to a lower class and the increase in incidences of blackleg in ware crops especially when grown in relatively warm climates. D. solani more effectively infects potato plant tissues after inoculation and expresses symptoms at lower number of bacteria present than the other Dickeyas. Some researchers attribute the shift to higher temperatures associated with climate change. The bacteria may also benefit from the increased erratic rainfall patterns whereby fields are flooded periodically more frequently. P The blackleg disease spreads from crop to crop primarily through the seed tubers. Tolerance levels in pre-basic seed are zero in most countries, in basic seed the tolerance level is about 0.5 % in many countries but still zero in the Netherlands whereas Scotland tolerates some Pectobacterium but zero percent Dickeya. An infected seed tuber may yield a daughter plant that shows a blackening of the lower part of the stem and wilting of the leaves. The mother tubers usually rot completely releasing enormous amounts of bacteria in the soil. Dr J.M. van der Wolf at Wageningen UR specializes in the disease (e.g. Phytopathology 100 page 134 and 1128) and found proof in scientific literature that P. atrosepticum especially spreads when rotten tubers contaminate healthy otato growers are used to the name Erwinia referring to E. POTATO R ESEARCH seed tubers during harvesting and grading when tubers are bruised. Also during growth in the field the tubers may get infected by bacteria invading the lenticels that especially open when the soil is very moist and bacteria easily spreads with flowing water in the field. It is observed that plants neighboring an infected plant often contain greater numbers of bacteria and are at greater risk of showing symptoms. It is known that Dickeya infects tubers from the outside by bruising and lenticels infection. Mechanical haulm destruction is another risks of spreading bacteria from plant to plant and from infected plant to the soil. Blackleg bacteria survive for about three months in the soil, a period that is not much influenced by soil type, temperature or moisture. So it is assumed that when a soil at planting is devoid of (volunteer) potatoes the seed is the only source of inoculum. The fact that blackleg bacteria are found at highest density at the stolon end shows that infection can also take place from the mother tuber through the stem and stolons within the plant to the daughter tubers. Longtime it was assumed that – beside infection through the lenticels – blackleg bacteria only may invade the plant through roots damaged by machines or pests. Van der Wolf and colleagues recently showed that the bacteria can also enter undamaged roots. To this end they tagged a strain of Dickeya with a gene producing a fluorescing protein and watered pots with potato plants with its suspension. Upon invasion of the roots the spread of these fluorescing bacteria could be followed with a microscope from root to stem to stolon towards the tubers. The bacteria only marginally infected plants quicker when the roots were damaged by cutting them. This study shows that besides avoiding spread of blackleg by assuring that minimal damage occurs at field operations, harvest and handling additional risk should be reduced by low soil inoculum realizing the disease easily enters even undamaged roots. Anton Haverkort anton.haverkort@wur.nl Potato World 2010 • number 4 29 Pagina 28
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