The susceptibility of tubers of different potato cultivars to soft rot by Erwinia carotovora subspp. j'rrjseptica and carotovora was assessed in 3 years by two methods. In one method, whole tubers inoculated at wounds with either bacterium were incubated under anaerobic conditions for 5 or 0 days at 15°C. In the other method, wounds made in tuber slices were allowed to heal ornot, Defore inoculation with different concentrations of each bacterium and were then incubated under aerobic conditions for 3 days at 15°C, Most cultivars gave consistent reactions in repeated experiments using the same method, but there was some seasonal variation. A few cultivars were .-onsistently susceptible (Klondyke and Manna) oi resistant (Drayton) in both methods but others gave completely contrasting results (Record), In both methods and with all cultivars more rotting was caused by subsp. atroseptica than by subsp, carotovora because of the temperature of incubation.

S U M M A R YThe mechanisms of control of potato common scab (Streptomyces scabies) by irrigation were investigated in 5 years by studying the growth and surface microflora of young potato tubers developing in wet or dry soils. Direct examination of the tuber surface by light or scanning electron microscopy showed that the newly formed internodes near the apex, which carry stomata, had a very sparse microflora. In dry soil the older internodes, which carry lenticels at a susceptible stage, were increasingly colonised by actinomycete hyphae and discrete, although sometimes large, bacterial colonies. In wet soil, actinomycetes were rarely seen on tuber surfaces but bacteria were generally scattered over them, differences not always shown by isolation from periderm pieces onto water agar.When dry scab-infested soil was wetted, scabs did not subsequently develop on the two youngest tuber internodes (A-1, A-2), which may be an example of disease escape rather than inherent resistance of the stomata. The scab control achieved in wet soil was probably caused by some form of microbial antagonism, but whether through competition or antibiosis was not established.

23 Factors affecting the field infection of potato tubers of different cultivars by blight (Phytophthora infestans) I N T R O D U C T I O NLate blight (Phytophthora infestans (Mont.) de Bary) remains one of the most important diseases limiting world potato production and is costly to control. Breeders still seek for durable resistance but reliable screening tests, especially for tuber resistance, are elusive (Schober & Hoppner, 1972; Langton, 1972), and too little is yet known of important factors operating in field epidemics. 24 D. H. LAPWOODSince 1968, the weather in successive seasons has not encouraged attacks of foliage and tubers at Rothamsted and elsewhere (Croxall & Smith, 1976), so tuber infection has seldom reached I % on unsprayed King Edward, in marked contrast to the early 1960s when it often exceeded 20% (Hirst & Stedman, 1973). As there is little evidence that our ability to control severe epidemics has increased, they must be expected to recur and it is appropriate now to review our existing knowledge. This paper first presents new data on tuber infection obtained during the blight years 1960-6 and then reconsiders the factors involved in tuber infection by collating these data with those obtained previously at Rothamsted (Hirst, Stedman, Lacey and Hide, 1965; Lacey, 1967b), and elsewhere. MATERIALS A N D M E T H O D SCultivars. In 1960, four tuber-susceptible cultivars, King Edward, Up-to-Date, Arran Banner and Ulster Ensign, were grown but subsequently the latter two were replaced by Majestic and Arran Viking, with moderately resistant tubers. Pentland Dell replaced Arran Viking in 1966, and there was no experiment in 1962.Plantiw and design. Plots, planted by hand (1960-3) or machine (1964-6) were of twelve rows, 28 in (71 cm) apart of twenty-five plants each 15 in (38 cm) apart.Plots were arranged in Latin square designs with two Majestic guard rows between each column of plots and on the outside edges of the experiment. Inoculation. A droplet of inoculum was placed on a single lower leaf temporarily

Majestic potatoes planted in soil naturally infested with Streptono,ces scabies were trickle irrigated to prevent infection of the tubers except during consecutive 7-day periods in the first eight weeks when tubers were developing (28 May to 23 July).During each interval without irrigation, lesions of scab affected an average of 4 internodes on the tubers but the later water was withheld, the closer to the apex of the tubers was the infection at final lifting. The tubers with most area scabbed were those unwatered between 11 and 18 june, the third week from tuber initiation, when the first-formed internodes, which expand more than later-formed ones, became infected.

Irrigation applied early during tuberization significantly decreased scab on tubers of Majestic, King Edward and Record, but not on Pentland Crown, a resistant cultivar, where tubers had little scab even from the driest soil. There was little benefit from keeping soils wetter than 0.6 in (15.24 mm) soil moisture deficit (s.M.D.) for more than 3 weeks after tuber initiation, and irrigation to 0.8 in (20.32 mm) S.M.D. gave economic control. Irrigating only after the 'marble stage' increased yield but the tubers were scabbed. Irrigation did not alter the rate eyes (nodes) separated from the apical bud but speeded the swelling of tubers.