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In N. Uruguay two potato (Solanum tuberosum L.) crops are grown per year, one in the fall and the other in spring. In each season physiologically young seed tubers are planted, resulting in relatively slow crop emergence and low yields. The aim of the current research was to determine the effects of physiological ageing of seed tubers on the dynamics of canopy growth, radiation interception and radiation use efficiency (RUE) of crops in the short seasons of Uruguay and to identify opportunities for increasing yield. Experiments were conducted on cv Chieftain over three years from 2000 to 2002. Prior to planting seed was stored at different controlled temperatures to generate physiological ages (measured in thermal time) ranging from 0 (the youngest) to 1200 oC days (the oldest). Ageing led to earlier crop emergence (3-9 days depending on the year), but its potential benefits on radiation interception, dry matter production and yield were negated by an earlier canopy senescence and often smaller canopy size. There was no consistent effect of ageing on RUE although the harvest index was increased. Leaf tagging showed that leaf appearance and senescence was advanced by ageing and the lifespan of individual leaves was reduced. Leaf lifespan did not appear to be related to the sink demand by tubers for assimilate, because removal of tubers at the start of bulking had no effect on lifespan. Maintaining canopy structure during senescence led to a small increase in yield. Although physiological ageing did not result in a consistent increase in yield in either season, use of older seed may provide an insurance against the risk of adverse weather conditions late in the season terminating tuber bulking prematurely.

Thesis (M.Sc.(Zoology and Entomology))--University of Pretoria, 2008. / Summary in English. Includes bibliographical references.

Field experiments were conducted in two different growing environments to evaluate the spread and movement of Dickeya dadantii. A procedure to inoculate seed potatoes with Dickeya dadantii was developed to use during this study. Spread of Dickeya dadantii from inoculated potato seed to healthy potato seed during the handling, cutting and planting procedures was not detected at either location. Spread of Dickeya dadantii from inoculated seed to surrounding progeny tubers in the field was documented in both locations. In Florida, 33% of progeny tubers tested positive for Dickeya using PCR, and in North Dakota, 13% of the progeny tubers tested positive. Stunting was observed in plants grown from Dickeya dadantii inoculated seed tubers in North Dakota, but not in Florida. These results indicate that Dickeya dadantii may spread during the seed handling and cutting processes and can spread in the field from infected seed tubers to progeny tubers.

Potato (Solanum tuberosum L.) varieties Atlantic and Granola are widely grown in Indonesia. The optimal method of cultivation in the tropics, due to the susceptibility of cut seed for disease, is by small (20 to 55 g) whole seed potatoes. However, the variety Atlantic produces mostly large tubers, which are not suitable for planting as whole seeds. Although Granola produces a reasonable proportion of small tubers it still produces a few in the larger size grades and there is no fresh market in Western Australia for the larger tubers for this variety. The aim of this study was to develop methods to be used in Western Australia that improve the yield of small seed potatoes for export to Indonesia. The influence of seed-potato storage duration (at 4°C) on subsequent stem growth was assessed after 30 days growth in a glasshouse (22°C/18°C, day⁄night). Seed potato storage for 22-28 (Atlantic) and 24-30 (Granola) weeks resulted in development of higher numbers of stems. A series of field experiment were designed to increase yield of small tubers. Apical sprout removal in Granola, but not Atlantic, increased the number of stems (by 27%), yield of 20-55 g potato (by 32%) and total yield (by 17%). Application of herbicide (paraquat + diquat) at low concentration during early tuber initiation decreased total yield in Atlantic (by 14%) and Granola (by 16%). Treating whole seed potatoes with carvone vapor two weeks before planting had no influence on stem or tuber number in both Atlantic and Granola but in Atlantic only, the total yield was reduced by 12%. Spraying plants with paclobutrazol during early tuber initiation inconsistently influenced tuber number and yield between the two varieties and two experiments. The influence of gibberellic acid (GA3) on stem number, total tuber number, yield of 20-55 g tubers and total yield was investigated by dipping seed pieces in a GA3 solution (20 mg⁄L) two days prior to planting. In Atlantic, GA3 treatment increased stem number (by 147%), total tuber number (by 75%) and yield of 20-55 g tubers (by 330%) without influencing total yield. In Granola, GA3 treatment increased stem number (by 50%), total tuber number (by 15%), yield of 20-55 g tubers (by 21%) and total yield (by 10%) The influence of gibberellic acid application (20 mg⁄L) to seed pieces before planting increased the number of small tubers through increased stem number. The shift toward a greater proportion of small tubers, without reducing total yield, had a greater influence in Atlantic than that in Granola. Treatment of GA3 and paclobutrazol together decreased total yield compared to that of GA3 alone