POTATO HAULM REGROWTHS AND THE POSSIBILIT IES OF THEIR REDUCING

Exact small-plot trial established in the years 1990 to 1993 studied t he effect of regrowth as exerted on potato haulms by selected practice s after their premature destroy. In the first year we chose treatments of a trial and were finding a susceptibility of varieties to regrowth . Of the group of varieties Resy - very early, Karin - early, Eba - se mi-late, Resy and Eba were selected as susceptible to regrowth and wit h respect to their distribution in practice. Except the control, in wh ich the potato stand was left to ripen naturally, haulms were destroye d chemically or mechanically on a date given by the Central Control an d Testing Institute for Agriculture (UKZUZ) or a week later. In total, there were seven treatments, except control, in the trial (Tab. I). A s to the treatments, the stand was affected by biological preparation of planting material (N - unchitted tubers, planted directly from pota to store-house, P - chitted tubers stored at light with 1 cm germs and bases of roots) (Tabs III to V). The trial had four replications. Reg rowth was observed in ten-day intervals after destruction of haulms an d close before harvest in accordance with UKZUZ regulations. According to them, regrowth is meant as: a) each new shoot (stem, branching) at a length above 10 cm if a plant has minimum two shoots; b) shoots (st ems, branching) below 10 cm, and longer than 5 cm, if there are minimu m three shoots per plant. Except evaluation of regrowths according to these regulations we recorded each indication of regrowth which was re ckoned in the total number of regrowths. Occurrence of scurf of potato es was observed along with its relationship to regrowth. For regrowth of stands themselves after their desiccation, except the quality of pe rformed control (mechanical destruction of haulms, chemical spraying w ith Region with diquat as active agent), moisture or volume and distri bution of precipitation played a decisive role in the time of control and after it. Weather pattern (sum of precipitation in different month s and average temperature) prevailing in individual years of the trial is shown in climatograms (Figs 1 to 4). Precipitation, average temper ature and length of sunshine are presented in Tab. II. Precipitation d uring desiccation and following it is analysed in details, as well as the number of regrowths in different varieties in each year of trials. The effect of soil moisture was more marked by inclusion of the treat ment with irrigation (5). Higher soil moisture increased regrowths (Ta bs III and IV). In 1993, the year in which this irrigation was not per formed in the required term, its effect on regrowths (Tab. V) cannot b e considered. Biological preparation of planting material (chitting) w hich decides upon the physiological age of stands reflected in the num ber of regrowths in all three years. In two years (1991 and 1993) regr owths decreased highly significantly in those stands in which haulms w ere desiccated with a week delay (treatment 2). The results were congr uent provided that physiologically older stand's recovery ability of h aulm production is not so high. Remaining practices with an aim to red uce regrowths had not unambiguously positive impact [reduced N rate fr om 120 (treatment 6) to 80 kg.ha(-1) (treatment 1); split of rate of N .ha(-1) - 40 kg of N.ha(-1) during planting, 40 kg of N.ha(-1) per sta nd (treatment 7); spraying of the stand with Cultar with active agent paclobutrazol in a lead before desiccation of stand (treatment 4)]. Me chanically destroyed haulms in a given time of desiccation with subseq uent chemical treatment (treatment 3) did not give better results in r egrowths regarding chemical destruction carried out in this term (trea tment 1). Plants infected with scurf of potatoes had a tendency to hig her regrowths.