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Fred T. Davies Jr., Constantino M. Calderón, and Zosimo Huaman

International Potato Center (CIP) for supplying the tissue-culture produced minitubers and Efrain Malpartida for supplying the Italian rye seed for trap cultures. We thank Neil Anderson, Reforestation Tech International (RTI) for supplying the Glomus

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Roger L. Vallejo, Wanda W. Collins, and Rocco D. Schiavone

requirements for a PhD by R.L. Vallejo and was entirely supported by the International Potato Center, Lima, Peru. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be

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Robert W. Goth, Ernest W. Goins, and Kathleen G. Haynes

Potato leafroll virus (PLRV) is a serious aphid transmitted virus disease of potato (Solanum tuberosum L.). Field observations suggest that the cv. BelRus is tolerant to PLRV. Greenhouse grown BelRus and PLRV susceptible potato cvs. Green Mountain and Katahdin were tested for PLRV with enzyme linked immunosorbant assay (ELISA) and subsequently infested with PLRV infected green peach aphids (Myzus persicae). ELISA was used to test leaves from the top, middle and bottom portion of the plants at 7 day intervals beginning 7 days after aphid infestation. PLRV was detected in all tested locations of the Green Mountain and Katahdin plants 21 days after inoculation. In BelRus, throughout the 11 week test, PLRV was detected predominantly in the top portion of the plants and at low titres. These results suggest that tolerance to PLRV infection in the cv. BelRus may be due to suppression of virus replication.

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Zana C. Somda and Stanley J. Kays

The effect of the plant density (15, 30, 45 × 96-cm spacing) on the branching pattern `Jewel' sweet potato [Ipomoea batatas (L.) Lam.] was determined bi-weekly for 18 weeks. Plant density effects were significant for the number of branches formed and timing of branch formation. Plant density did not affect the type of branches formed (e.g., primary, secondary, and tertiary), but did alter the timing of induction during the growing season. By the end of the growing period, the ratios for the number of primary to secondary branches were 1.5:1, 1.3:1, and 0.6:1 at the 15-, 30-, and 45-cm spacing, respectively. Few tertiary branches were formed, but were present on some plants at each spacing. Tertiary branches most commonly occurred on plants at the widest spacing. While the number of branches per plant was highly plastic and inversely related to plant density, nodes per branch and internode length were not significantly affected. Average internode length per branch decreased with descending branch hierarchy (i,e., main stem < primary branch < secondary branch). `Jewel' sweet potato responded to increased space available largely through production of additional branches with the modification of branching pattern increasing as the season progressed.

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Wayne C. Porter

Oxyfluorfen was evaluated for weed control in sweet potatoes. In 1989, applications were made overtop transplants immediately after transplanting. The 1990 applications were made just prior to transplanting. Oxyfluorfen applied post-transplant at 0.38 lb ai/A and greater rates caused a significant reduction in crop vigor. A 1.0 lb ai/A rate of oxyfluorfen reduced crop vigor when applied pretransplant. All rates of oxyfluorfen controlled Brachiaria platyphylla, Digitaria sanguinalis, Cyperus iria, and Sesbania exaltata. Oxyfluorfen rates of 0.5 lb ai/A and greater were needed to consistently control Sida spinosa and Echinochloa crus-galli. Mollugo verticillata was controlled at all rates in 1989 but not controlled at all in 1990. Yields of all grades of sweet potato roots from plots treated with oxyfluorfen were not different from yields from plots treated with currently labeled herbicides. However, in 1989 yields from all oxyfluorfen-treated plots were lower than yields from the hoed check. In 1990, plots treated with oxyfluorfen at 0.25 or 0.38 lb ai/A had lower yields of No. 1 grade roots than the hoed check.

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Martha Maletta, Melvin Henninger, and Kristian Holmstrom

Control tactics for potato leafhopper (Empoasca fabae) in certified organic potato (Solanum tuberosum) production were evaluated in 2003 and 2004. The 2004 split plot trial also compared silver plastic mulch culture with bare ground culture. The trials were conducted at the Rutgers Snyder Research and Extension Farm's certified organic fields, and production practices conformed to the standards of the National Organic Program (NOP). `Superior' potato was grown both years. Potato leafhopper (PLH) controls evaluated were: kaolin, pyrethrin, pyrethrin plus kaolin (2003), pyrethrin with silicon dioxide (2004), and silver plastic mulch (2003). Pyrethrin, pyrethrin plus kaolin, and pyrethrin with silicon dioxide reduced PLH nymph counts and PLH damage (hopperburn) ratings compared with the untreated check (UTC). Kaolin did not reduce nymph counts or hopperburn ratings. In 2003, nymph counts and hopperburn ratings were higher in the mulch treatment than in the UTC, yet the mulch treatment produced higher yield than the UTC. In 2004, mulch culture increased total and marketable yield compared with bare ground culture when PLH was controlled. Nymph counts and hopperburn ratings were higher until mid-July in the mulch plots than bare ground plots with the UTC and kaolin treatments. Controlling PLH and using plastic mulch culture significantly increased organic potato yields and tuber size. Marketable yields from the UTC were less than the New Jersey average of 275 cwt/acre for conventionally grown potato: yield was 38% of average on bare ground and 68% of average on mulch in 2003; 33% of average on bare ground and 38% of average on mulch in 2004. Reducing hopperburn with pyrethrin on plants grown on mulch (2004) resulted in marketable yield that was 75% of the New Jersey average.

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Barbara J. Daniels-Lake, Robert K. Prange, Sonia O. Gaul, Kenneth B. McRae, Roberto de Antueno, and David McLachlan

In 2001, several million kilograms of fresh and processed potato products from the Annapolis Valley of Nova Scotia, Canada (i.e., most of the local crop) was rejected by consumers because of a strong “musty” “off” flavor and odor (OFO). As a

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S.S. Snapp and A.M. Fortuna

Growers lack practical decision aides that accurately predict nitrogen (N) credits for organic sources to adjust fertilizer rates. The simulation model, DSSAT, was used to predict N supply in relationship to N demand in irrigated potatoes (Solanum tuberosum). Tuber yield and soil inorganic N levels were substantially higher in the simulations than in field experiment observations, indicating the need for model improvement. DSSAT was successful at predicting relative mineralization rates and potato N uptake for different organic and inorganic N source combinations. Interestingly, both simulation and field experiment observations indicated that combining a high quality organic manure at 5000 lb/acre (5604.2 kg·ha-1), total applied N 250 lb/acre (280.2 kg·ha-1), and a fertilizer source of N 160 lb/acre (179.3 kg·ha-1) markedly increased yields and lowered leaching potential. Simulated tuber yield for the combined treatment was 660 cwt/acre (74.0 t·ha-1) with 48 lb/acre (53.8 kg·ha-1) inorganic-N in the profile at harvest, whereas the highest simulated N fertilizer response was to 235 lb/acre (263.4 kg.·ha-1), which produced 610 cwt/acre (68.4 t·ha-1) with 77 lb/acre (86.3 kg·ha-1) inorganic-N in the profile at harvest. The synchrony of N release and uptake for combined manure and fertilizer treatments may explain the efficient N uptake observed. Common soil types and weather scenarios in Michigan were simulated and indigenous soil N mineralization was predicted to be 6 lb/acre (6.7 kg·ha-1) inorganic-N in the topsoil at planting, similar to observed levels. The increasing aeration associated with a sandy versus a sandy loam soil only slightly increased the predicted rate of mineralization from organic inputs. Simulated soil inorganic N levels with different organic inputs was modestly increased in a warm spring [4.5 °F (2.50 °C) over normal temperatures] compared to a cool spring (-4.5 °F less than normal temperatures). For Michigan irrigated potato systems, DSSAT simulations indicate that the most important factor determining inorganic N supply will be the quality and quantity of organic inputs, not environmental conditions.

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Roland E. Roberts and Michael G. Hickey

Texas processing potato growers want high tuber yield and soluble solids with optimal nitrogen (N) fertilization to avoid leaching N into underground water. A 3-year on-farm study demonstrated petiole and soil testing methods for N enabling growers to apply N at rates and times for maximum yield with acceptable specific gravity. For example, a FL-1553 crop received 160 N kg/ha in irrigation water spread over 126-day season. Sampling every 2 weeks from early vegetative stage to harvest showed petiole N of 22,000 ppm on day 40; 6,000 ppm on day 54; 3,000 ppm on day 68; 7,000 ppm on day 96; and 2,000 ppm by day 110. At harvest total tuber N = 1.12%; tuber specific gravity = 1.081 (17.1 % FritoLay solids); and processor acceptable yield = 304 q/ha. The crop removed nearly all applied N.

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Noël Pallais, José Santos-Rojas, and Rosario Falcón

Sexual potato (Solanum tuberosum L.) seeds require many months of afterripening in dry storage to completely lose dormancy and germinate readily at >25C. We examined the relationship between storage temperature and seed dormancy, as assessed by the percentage of germination after 4 days. Two F1 hybrid lots of `Desiree' × 7XY.1 were used; one seed lot was produced by carefully removing half of the developing tubers from the mother plant during seed development, and the control remained undisturbed. Seeds were stored with 3.4% moisture (dry-weight basis) at 10, 20, 30, 40, and 50C and were tested eight times during 29 months for daily germination at 27/40C (21/3 h) for the first 8 days, followed by 6 days at 17C. After 29 months of storage, final germination was <97% only when control seeds were stored at 50C, in which germination was 72%. Germination after 4 days increased curvilinearly with increasing storage temperature, and both seed lots similarly lost dormancy (germination >90%) after 10 months at 40C. Optimum germination levels were maintained after 29 months at 40C. Seeds stored at 50C never completely lost dormancy, and after 7 months of storage, germination at 4 days gradually decreased to zero. Dormancy was eventually lost after 29 months in most seeds stored at <40C, and differences between seed lots suggest that removing tubers from the mother plant increased dormancy. We conclude that dry potato seeds can be safely afterripened at temperatures up to 40C; lower temperatures slow the rate of dormancy loss, and higher ones are detrimental to seed quality.