Variation in Aggressiveness of Canadian Isolates of Phytophthora infestans as Indicated by Their Relative Abilities to Cause Potato Tuber Rot

Studies were conducted in 1995, 1996, and 1997 to determine the ability of Canadian isolates of Phytophthora infestans to cause tuber disease in a variety of potato cultivars. Most isolates of recently introduced multilocus genotypes (US-7, US-8, g11, g26, g29, and g40) were more aggressive on tuber tissue than isolates of the traditional US-1 genotype, based on surface necrosis (SN), lesion depth (LD), and compound aggressiveness index (CAI = SN × LD) components. Other multilocus genotypes (g30, g41, g42, and UN-3) were similar in aggressiveness to US-1. The g11 (US-11) genotype consistently displayed aggressiveness that was intermediate to that of the US-8 and US-1 genotypes. Variation also occurred within a multilocus genotype, and an isolate of the US-8 genotype from New Brunswick was consistently less aggressive than other US-8 isolates. Cvs. Dorita and Island Sunshine were consistently the most resistant to infection, regardless of isolate, relative to the more susceptible responses of Green Mountain, Russet Bur-bank, Kennebec, Superior, Shepody, Red Pontiac, Sebago, and Bintje. Even so, isolates of the newly introduced US-8 genotype were able to cause significantly more disease on these resistant cultivars than isolates of the US-1 genotype. The predominant genotypes currently found in potato production areas in Canada (US-8 and g11) have higher fitness on tuber tissue than displaced, preexisting strains (US-1).

The Effect of Jasmonic Acid on in Vitro Nodal Culture of Three Potato Cultivars

Jasmonates are a group of native plant bioregulators that occur widely in the plant kingdom and exert various physiological activities when applied exogenously to plants. We investigated the effect of free jasmonic acid (JA) on stem and root growth and tuberization of potato in vitro nodal culture. Nodal cuttings of three potato cultivars, Norchip, Red Pontiac, and Russet Burbank, were cultured in 2.5 × 15 cm test tubes containing either nodal culture (MS with 2% sucrose) or tuber-inducing (MS with 8% sucrose and 11.5 μm kinetin) medium. The media were supplemented with JA at 0, 0.1, 0.5 1.0, 5.0, 10.0 and 50 m. The cultures were maintained under a 16-hour photoperiod at 24°C for 6 weeks. Potato cultivars showed different sensitivities to JA in stem growth. Norchip is the most and Red Pontiac the least sensitive cultivar. On the nodal culture medium, stem length of Norchip was promoted at 0.1–5 μm, and inhibited at 10–50 μm of JA, but that of Red Pontiac was promoted by JA at all concentrations tested. The number of nodes increased significantly on media with JA than that on medium without JA. The number of adventitious roots did not, but the lateral roots increased significantly when JA was added to the medium. On tuber-inducing media, stem length and node number did not appear to be affected by addition of JA to the medium. The number of axillary shoots increased significantly on the media with low concentrations of JA (0.1–5 μm). No microtubers formed on both media from all three cultivars in 6 weeks.

ALTERAÇÃO DOS NÍVEIS DE AÇÚCARES REDUTORES E SACAROSE DURANTE O ARMAZENAMENTO DOS TUBÉRCULOS DE BATATA DE NOVOS CLONES

Tubérculos de três novos clones de batata e da cv. "Red Pontiac" foram armazenados sob temperaturas de 1ºC, 8ºC e 20ºC. Os tubérculos armazenados sob condições de baixas temperaturas foram mantidos por 30, 60, 90 ou 180 dias e posteriormente recondicionados por 10 dias a 20ºC. Nenhuma diferença significativa foi detectada entre os clones sob as condições estudadas. O armazenamento a 1ºC estimulou o acúmulo de açúcares redutores e sacarose, principalmente nas primeiras semanas. O recondicionamento a 20ºC reduziu estas concentrações.

DETECTION AND OCCURRENCE OF PRIMARY INFECTION BY POTATO VIRUS S OF DISEASE-FREE PLANTS

Using enzyme-linked immunosorbent assay (ELISA) and potato plantlets of cultivars Green Mountain, Red Pontiac, Russet Burbank and Shepody, potato virus S (PVS) was detected in primary infected plants under field conditions. Potato virus S was detected 3 wk postinoculation (p.i.) from the field-grown plants as compared to 2 wk p.i. from greenhouse-grown plants. Potato virus S concentration in infected plants remained high throughout the growing season. The ELISA test using dormant tubers was not reliable because A405 nm values overlapped between infected and healthy tubers. Exposure of healthy plants to PVS sources in the field resulted in high infections with PVS by the 9th wk of growth. This pattern of infection suggests involvement of aerial vectors in the transmission of PVS under conditions of the Atlantic region of Canada.Key words: Current year infection, ELISA, dormant tubers, spread of PVS

Weed Management in Two Potato (Solatium tuberosum) Cultivars Using Tillage and Pendimethalin

Tillage and pendimethalin either alone or in combination were compared for weed management in two potato cultivars. The best weed management program consisted of a combination of pendimethalin and tillage. Pendimethalin alone did not adequately control weeds. Multiple cultivations gave better control than pendimethalin alone but not as good as pendimethalin and cultivation combined. The net returns using tillage, pendimethalin, and tillage plus pendimethalin for weed control averaged –$86, –$208, and $ 143/ha, respectively. Independent of their effect on weeds, cultivation had a significant negative effect and pendimethalin a significant positive effect on potato yields. Each cultivation reduced yields by 1.7% and pendimethalin increased yields by 4%. The cultivar ‘Red Pontiac’ was a better competitor with weeds than ‘Red Norland’.

Glyphosate Uptake by Suspension-Cultured Potato (Solanum tuberosumandS. brevidens) Cells

Characteristics of glyphosate [N-(phosphonomethyl)glycine] uptake for as long as 3 days by plant cells were studied using suspension-cultured potato (Solanum tuberosumL. cv. ‘Red Pontiac’ andS. brevidensPhil.) cells. An experimental system was developed wherein uptake was assayed in a reduced salt medium that minimized cell growth yet maintained cell viability. Glyphosate uptake was nonlinear over time; an immediate rapid uptake phase was followed by uptake at a decreased rate that continued through 72 h. Glyphosate uptake was the same in both an experimental uptake medium (reduced salt and sucrose, no hormones) and in the complete growth medium used for routine cell growth. The pattern of glyphosate uptake was similar with both Red Pontiac andS. brevidenscells, though uptake was greater in Red Pontiac cells. Glyphosate uptake was not inhibited by 1 mM CaSO4, MgSO4, MnSO4, or ZnSO4. Rather, uptake was inhibited by the absence of salts at 8 h, and ZnSO4and MnSO4increased uptake. Similarly, increasing concentrations (0 to 5 mM) of CaSO4and CaCl2did not inhibit uptake, whereas the absence of either salt of Ca2+decreased uptake. Increasing concentrations of (NH4)2SO4(0 to 150 mM) did not affect glyphosate uptake, but (NH4)2SO4at 50 mM and above was toxic to the cells.

Crop Response and Arsenic Uptake Following Soil Incorporation of MSMA

Five crops were grown for three seasons following a single soil-incorporated application of MSMA (monosodium methanearsonate). Crop sensitivity to soil arsenic decreased as follows: snap beans (Phaseolus vulgarisL. ‘Early Gallatin’ and ‘Tenderette’); rice (Oryza sativaL. ‘Dawn’ and ‘Starbonnet’); soybeans [Glycine max(L.) Merr. ‘Lee 68′]; potatoes (Solarium tuberosumL. ‘Red LaSoda’ and ‘Red Pontiac’); and cotton (Gossypium hirsutumL. ‘Stoneville 7A’). Over the 3-yr period, yield of snap beans was reduced 8, 14, and 85% below the untreated check where MSMA had been incorporated initially at 22, 45, and 269 kg/ha, respectively. Similar reductions in yield of rice were 18, 25, and 55%. Soybean and potato yields were significantly reduced only at the highest rate of MSMA while cotton yields were unaffected. Arsenic content in the edible portion of the crops decreased as follows: rice, snap beans, potatoes, soybeans, and cotton. The arsenic content in crops from the highest MSMA plots seldom exceeded twice that from the untreated plots.