Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 5. Type of resistance toP. infestans

Summary Plants of wildSolanum species (S. demissum (dms), S. verrucosum (ver), S. iopetalum (iop), S. brachycarpum (bra), and an unidentified species (unsp), from the natural habitat were inoculated in the laboratory withPhytophthora infestans race 0, to see if major genes for resistance were present, and with the most complex and aggressive race available in an attempt to reveal the level of general resistance. No plant with a susceptible reaction to race 0 was found in 120 plants ofdms, and 34 plants ofunsp. Plants with a hypersensitive reaction to race 0 and susceptible plants were found inver. When infected with a complex aggressive isolate of the fungus 73.5% of the WSS plants were susceptible. It was concluded that WSS in the area possess both complex race specific resistance and a high level of general resistance, whose effects cannot yet be separated.     

Biological factors influencing the transmission of potato leafroll virus by different aphid species

Summary Macrosiphum euphorbiae, collected in the field from potato plants infected with potato leafroll virus (PLRV), transmitted the virus to fewer potato plants in a field trial than did laboratory-rearedMyzus persicae. In the laboratory,M. persicae was the only efficient vector of PLRV fromPhysalis floridana seedlings, potato sprouts or excised leaves toP. floridana. Two clones ofM. euphorbiae and one clone ofAulacorthum solani transmitted PLRV from infected potato plants toNicotiana clevelandii as effeciently asM. persicae but a clone ofAphis gossypii was an inefficient PLRV vector. An isolate of PLRV, whichM. persicae transmitted inefficiently from potato toN. clevelandii, was also transmitted inefficiently byM. euphorbiae andA. solani.     

The development of mature plant resistance in four potato cultivars against aphid-inoculated potato virus YO and YN in four potato cultivars

Summary Field-grown potato plants of cvs King Edward, Record, Maris Piper and Désirée were inoculated on seven different dates during the growing season of 1987 and 1988 with either potato virus Y^O (PVY^O) or PVY^N, using three viruliferous peach-potato aphids (Myzus persicae) per plant. In each cultivar, the proportion of progeny tubers infected with PVY^O or PVY^N was high in plants inoculated during the four weeks following emergence, the proportion declining to zero or close to zero in the subsequent 4–6 wks.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 2. Distribution ofPhytophthora infestans

Summary An area of 15 000 ha on the Western slopes of the volcano Nevado de Toluca was surveyed during 1982–1986. The occurrence of late blight (Phytophthora infestans) was studied in 1986 along five long (7–10 km) transects from an altitude of 2900 m to 3900 m, the upper limit ofSolanum demissum (dms) and in all years in some other sites. Late blight was confined to scattered pockets, 100–500 m in diameter, separated by several kilometers. In 789 colonies of wildSolanum spp., each consisting of 4 to >100 plants per colony ofdms, verrucosum (ver), iopetalum, brachycarpum, × edinense, stoloniferum or one unidentified species, symptoms were found in 10% of the colonies or less than 1% of the plants in each colony. Plants had few (1–2) lesions which did not affect vigour. No infected tubers were found. Most (64%) of the infected colonies were ofdms, the predominant species (69%). Colonies ofver were less frequent (11%) and 17% were infected. No infected plants were found in the unidentified species, thought to bedms×ver.     

Effect ofErwinia carotovora subsp.atroseptica alone and withAlternaria solani orVerticillium dahliae on disease development and yield of potatoes in Israel

Summary Tubers of 17 cultivars were inoculated withErwinia carotovora subsp.atroseptica (ECA) and planted in 1986 and 1987 in the Negev in randomised plots in three blocks where eitherVerticillium dahliae orAlternaria solani were present or where both were absent. ECA reduced the yield in the control block and exacerbated the yield losses in the disease blocks, but the intensity of symptom expression did not always accord with the extent of yield loss. ECA had a greater effect on plants infected withA. solani than on those withV. dahliae. It was possible to categorise the cultivars as resistant, intermediate or susceptible with respect to ECA and early blight, but resistance toV. dahliae could not be differentiated from foliage maturity.     

Resistance toSpongospora subterranea in tuber-bearing and non tuber-bearingSolanum spp.

Summary The incidence of infection withSpongospora subterranea was studied in the non tuber-bearingSolanum brevidens andS. etuberosum and the tuber-bearingS. acaule, S. sucrense andS. tuberosum cvs Olympia and Pito. Shoot cuttings were grown in soil naturally infested withS. subterranea, or the roots were inoculated with a zoospore suspension. Logit models were used to analyse the data. The incidence ofS. subterranea was higher in plants inoculated with zoospore suspension than in those grown in infested soil (odds ratio (OR) 10.65). Ageing the inoculum reduced the incidence of infection in the plants (OR 0.30) without altering the interspecific differences. The ORs of infection (compared to cv. Olympia) were 0.07, 0.29, 0.60 and 2.88 forS. acaule, S. sucrense, S. brevidens andS. etuberosum. OnlyS. acaule was significantly more resistant to infection than cv. Olympia. No infection was detected in cv. Pito.     

Pathogenicity ofFusarium oxysporum f.sp.tuberosi isolates from tubers and potato plants

Summary A study was carried out on eightF. oxysporum f.sp.tuberosi isolates from northern Italy. Six were from rotten tubers and two were from weak, but not wilted plants from seed-growing areas. Greenhouse and laboratory tests were done to assess the pathogenicity and potential spread of the fungus within the plant. All isolates caused dry rot on inoculated tubers. They were also able to infect vascular tissues through roots and leaves, but none caused wilt, stemend rot or other symptoms.F. oxysporum is often not visible in the field, but its propagules can increase under favourable conditions. It may therefore become a potential danger during potato storage.     

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 4. Plant development in relation to late blight infection

Summary Development of plants ofSolanum demissum (dms) andSolanum verrucosum was followed in three sites at altitudes of 2950, 3250 and 3500 m, and the incidence of late blight and physiological races determined. No plants were found that had grown from true seed although berry formation was abundant indms. Tubers sprouted during the dry period and most shoots emerged at the beginning of the rainy season in late April. Emergence continued for about two months indms, and new plants were later formed from stolons, so that plants of different ages were present at the same time. Late blight was first found in cultivated fields adjacent to the site at 2950 m, and three weeks later lesions were found in wildSolanum species (WSS) simultaneously at all sites. Few plants per colony of WSS were infected and none was seriously damaged. Complex races (6–10 virulence genes) were found early in the season at one locality.     

Systemic induced resistance to late blight in potato by treatment with salicylic acid andPhytophthora cryptogea

Summary Tests for systemic induced resistance (SIR) to late blight were performed with different potato cultivars in field trials and in growth chamber experiments. The non-pathogenPhytophthora cryptogea (Pc) and salicylic acid (Sa) were used as inducer agents. In the field, plants were naturally infected byPhytophthora infestans while the indoor plants were inoculated. The degree of SIR obtained varied with the cultivar used and also with the type of inducer. For example in the field the susceptible cv. St. Cecilia had enhanced resistance with Pc whereas Sa caused increased susceptibility. Inbred resistant cvs acquired increased resistance after treatment with Sa. In a separate pilot experiment in a growth chamber, Sa at two different concentrations was injected into potato seed tubers before planting. The higher Sa concentration increased resistance in genetically resistant cultivars while the lower Sa concentration brought about increased susceptibility.     

Effect of haulm treatments on the formation of microsclerotia ofVerticillium dahliae Kleb. on potato

Summary In four pot experiments, potato plants of cv. Element were artificially infected withV. dahliae. At an early and a late harvest haulms were killed chemically, by burning or by various other treatments, including cutting them into pieces of different lengths and keeping the debris on the soil surface or covering with soil. After 4 weeks the plant material was air-dried and the number of microsclerotia per mg was determined. At the early harvest, in two experiments, the chemical treatment yielded more microsclerotia than the cutting treatments. Covering colonised haulm tissue with non-sterilised soil was effective in inhibiting microsclerotia formation. Shorter haulm pieces led to fewer microsclerotia at the later harvest if the material was kept on the soil surface. The variation in microsclerotial yield and in treatment effects among the different experiments was large.     

A glasshouse progeny test for resistance to gangrene (Phoma foveata)

Summary Tubers were obtained from 80 glasshouse-grown seedlings of each of twelve progenies, inoculated withPhoma foveata Foister, and the mean percentage of infected tubers recorded. There was a good correlation (r=0.798) between this assessment and that made previously on field-grown tubers of eleven of the progenies. A gangrene test on glasshouse-grown tubers is suggested as a more rapid means of identifying progenies with useful levels of resistance.     

Detecting resistance to the root-knot nematodesMeloidogyne hapla andM. chitwoodi in potato and wildSolanum spp.

Summary Various methods of screening for resistance to root-knot nematodes were compared and evaluated. Seedling populations ofSolanum spp., grown in clay pots and plastic tubes with silver sand and inoculated with juveniles ofMeloidogyne chitwoodi andM. hapla, showed large differences in the number of egg masses on roots 7 weeks after inoculation. The differences were reproducible when re-testing was done with cuttings and plants from tubers. No resistance toMeloidogyne spp. was observed with ten potato cultivars when grown in clay pots, plastic tobes or closed containers. Plants from tubers in growth pouches developed a large two-dimensional root system, and after inoculation with juveniles the infection process could be observed over 8 weeks. A method of infecting potato tuber tissue withMeloidogyne is described, using tuber slices in Petri dishes as a potential screening test for tuber resistance.     

The effect of temperature on the development of wilting and on progeny tuber infection of potatoes inoculated with South African strains of biovar 2 and 3 ofPseudomonas solanacearum

Summary Potato plants, after inoculation with a South African strain of biovar 2 or a strain of biovar 3 ofPseudomonas solanacearum were grown in growth chambers at temperatures ranging from 14/16°C (dark/light) up to 25/30°C. When grown at 14/16°C or higher, plants inoculated with the biovar 2 strain developed wilting symptoms and internal symptoms on progeny tubers and the disease was transmitted by the progeny tubers. When the plants were inoculated with the biovar 3 strain, wilting symptoms developed from 18/20°C, internal symptoms on progeny tubers from 20/22°C and the disease was transmitted by progeny tubers from plants grown at 16/18°C or higher.     

Effects of contamination byErwinia carotovora subsp.carotovora andE. carotovora subsp.atroseptica of potato seed tubers and of cultivar resistance on blanking or nonemergence and blackleg development in Valencia,Spain

Summary High grade potato seed tubers of three cultivars were inoculated in Scotland by vacuum infiltration in suspensions ofErwinia carotovora subsp.carotovora and subsp.atroseptica in different combinations, and transported to Valencia, Spain. They were cut into two to three pieces and planted in January 1984, 1985 and 1986 in fields furrow irrigated before and after emergence. High incidences of blanking and blackleg were associated with seed inoculated with high numbers ofE.c. atroseptica alone or together withE.c. carotovora and not with high or low numbers ofE.c. carotovora alone.E.c. atroseptica predominated at emergence in March in rotting seed pieces which failed to produce a plant, regardless of the seed treatment, and in diseased stems in April but it was replaced byE.c. carotovora in May. The levels of blanking and blackleg after inoculating 16 cultivars with both erwinias were not correlated between years nor were they significantly different within years among most of the cultivars.     

Effects of crop rotation and granular nematicides on the incidence ofVerticillium dahliae Kleb. andColletotrichum coccodes (Wallr.) Hughes,in potato

Summary The incidence ofVerticillium dahliae andColletotrichum coccodes was studied in a crop rotation experiment on sandy soil from 1983 to 1986. Early in the growing season the percentage of stems infected byV. dahliae increased with increasing cropping frequency of potato, depending on the cropping sequence, and decreased with the application of granular nematicides. However, later in the growing season the initial differences decreased.C. coccodes infections of stems were not affected by these factors. The percentage of plants infected byV. dahliae closely correlated with the percentage of plants with wilt symptoms but this relationship was absent forC. coccodes. Early in the growing season, the more vigorous the plants, the greater the level ofV. dahliae infection. Soil infestation withV. dahliae increased with increasing cropping frequency of potato but was not affected by annual applications of granular nematicides.     

Plant-to-plant transmission of the bacterial ring rot pathogenClavibacter michiganensis subsp.sepedonicus

Summary Plant-to-plant transmission of the bacterial ring rot (BRR) pathogen,Clavibacter michiganensis subsp.sepedonicus (Spieck. et Kott.) Skapt. et Burkh. of potato (Solanum tuberosum L.), was studied in field trials over a three year period. Healthy and infected seed tubers were planted 35 cm apart. In one treatment, a subsurface barrier was placed between the healthy and infected seed tubers separating the root systems of neighbouring plants. In this treatment, none of 216 plants grown from healthy seed tubers was infected at harvest, as determined by indirect immunofluorescence antibody staining (IFAS) with monoclonal antibodies. In the other treatment, no subsurface barrier was used. In this treatment, two of 368 plants (0.5%) grown from healthy seed tubers were infected at harvest. It is concluded that plant-to-plant transmission may occur but at very low frequency, and is unlikely to play a significant role compared with the potential of transmission by shared potato handling equipment.     

Effect of a continuous hot water treatment of potato tubers on seed-borne fungal pathogens

Summary The viability of five pathogens was decreased by treatment with hot water when tested in vitro.Polyscytalum pustulans was most sensitive andRhizoctonia solani least sensitive. Potato tubers were exposed to 55°C for 5 min in a commercial continuous hot water treatment plant using naturally contaminated seed tubers and tubers which had been inoculated by dipping in comminuted cultures. The frequency of eyes colonised byP. pustulans, Helminthosporium solani, andR. solani was reduced to virtually zero and the effect persisted on tubers subsequently stored at 4°C and at 15°C for up to 16 wk. Results withColletotrichum coccodes were inconclusive. Treatment suppressedPenicillium spp. which, however, rapidly recolonised the eyes during storage, leading to higher contamination levels in the treated than in the untreated tubers. With tubers inoculated withPhoma foveata, good control was achieved when the incubation period before treatment was 10 d but not when the period was 42 d.     

Soybean inoculation: Dose, N fertilizer supplementation and rhizobia persistence in soil

The effect of rate of application of Sinorhizobium (Ensifer) fredii SMH12 or Bradyrhizobium japonicum USDA110 inoculants on grain yields of soybean [Glycine max (L.) Merrill] cv. Osumi was studied in a field experiment laid out in Southern Spain. All inoculant doses tested (10⁴, 10⁵, 10⁶ and 10⁷ rhizobia/seed) produced higher seed yields than those obtained in un-inoculated plots. Increments in nodule dry weight, seed yield and seed N content were observed when the number of rhizobia applied to seed increased from 10⁴ up to 10⁷. The addition of N fertilizer to inoculated soybean plants (50 kg N/ha applied at R1 or R4 stage) did not increase seed yields in comparison with treatments that were only inoculated. Survival of strains SMH12 and USDA110 was monitorized for three years in two different soils of Southern Spain after soybean inoculation. Rhizobia survival was varied dependent on the soil and the rhizobia strain used.     

Formation of microsclerotia ofVerticillium dahliae Kleb. on various plant parts of two potato cultivars

Summary Potato plants of cvs Element and Mirka were artificially infected withV. dahliae in two greenhouse experiments. Leaf blade, petiole, aerial stem, subterranean stem, stolon and root mass were separately harvested both when the canopy was still green and at maturity. After 4 weeks incubation, the plant tissue was air-dried and the numbers of microsclerotia per mg tissue and per plant were determined. The highest numbers of microsclerotia were observed in the haulm when harvest took place at maturity. Cultivar Element yielded significantly more microsclerotia in the haulm than cv. Mirka, whilst there were no cultivar differences in the microsclerotial production on subterranean parts. The petiole and the aerial stem contributed most to the total microsclerotial production, whereas roots were much more important for formation of microsclerotia than stolons.     

Penetration of potato tuber lenticels by bacteria in relation to biological control of blackleg disease

Summary In a glasshouse experiment three potato cultivars were grown to maturity in wet or dry compost. The number of lenticels per tuber was greater with larger tubers but was unaffected by the cultivar or soil moisture regime. The frequency of lenticel penetration by a water soluble stain, safranin O, remained high as tubers developed to maturity in wet soil. In dry soil penetration declined markedly as the tubers matured. When tubers, harvested just before the onset of haulm senescence, were inoculated withErwinia carotovora ssp.atroseptica, the frequency of lenticel rotting was less than the rate of stain penetration, indicating that additional factors were involved in the bacterial infection process. Inoculating mature tubers with bacteria and incubating them in anaerobic conditions frequently resulted in rotting of the lenticels. Comparable tubers incubated in aerobic conditions showed no rotting or invasion by bacteria. The results are discussed in relation to the biological control of blackleg.