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Juan C. Diaz-Perez, D. Bertrand, and D. Giddings

Tomato spotted wilt virus (TSWV) can cause serious damage to tomato, pepper, lettuce, and other crops. The virus is transmitted by several species of thrips. The objective of this study was to determine the effect of the time (t, days after transplanting) when TSWV symptoms first appeared on tomato plant size and fruit yield. Tomato (`Florida-47') plants were drip-irrigated and planted over black plastic mulch. The experiment was carried out in Tifton, Ga., during Spring 1999. High populations of thrips were detected since early stages of plant development, which resulted in a high incidence of TSWV. Plant fresh weight was significantly higher (r 2 = 0.632, n = 216) the later in plant development TSWV symptoms first appeared (i.e., with increasing t values). Total fruit production of individual plant linearly increased with increasing t values (r 2 = 0.664, n = 216). As with total fruit production, fruit marketable yield was also higher with increasing t values; however, marketable yield was significantly reduced even when plants were infected later in the season (t > 55 days after transplanting). Our results suggest that it is important to keep tomato plants free from TSWV as long as possible. This applies to both developing and developed plants.

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Juan C. Díaz-Pérez, William M. Randle, George Boyhan, Ronald W. Walcott, David Giddings, Denne Bertrand, Hunt F. Sanders, and Ronald D. Gitaitis

Sweet onions (Allium cepa L.) are typically grown on bare soil and irrigated with high-pressure systems such as sprinklers or center-pivots. The objective of this study was to determine the effects of irrigation system and mulch on bolting, bulb yield and bulb quality over 3 years. The experimental design was a split plot, where the main plot was irrigation system (drip or sprinkler) and the subplot was the type of mulch (bare soil, black plastic film or wheat straw). The results showed that individual bulb weight and bulb yields under drip irrigation were similar to those under sprinkler irrigation. Plants grown on bare soil had the highest total yield during the three seasons and among the highest marketable yield. There were no consistent differences in the bulb number or yield of plants on plastic film mulch compared to those of plants on wheat straw mulch. Plants on wheat straw mulch had reduced foliar nitrogen content. Variability in yields among mulches and seasons was partly explained by changes in seasonal root zone temperature and soil water potential. Total and marketable yields and weight of individual bulbs increased with increasing root zone temperatures up to an optimum at 15.8 °C, followed by reductions in yields and individual bulb weight at >15.8 °C. Onion bolting increased with decreasing foliage nitrogen content, with plants on wheat straw having the highest bolting incidence. Bolting also increased with decreasing root zone temperatures for the season. Total and marketable yields increased with decreasing mean seasonal soil water potential down to -30 kPa. Irrigation system and mulches had no consistent effect on the soluble solids content or pungency of onion bulbs.