Investigations into the Causes and Control of the Growth Arrestment Phenomenon of Sultana. II. Preliminary Experimental Investigations

Abstract

As a continuation of the investigation into the growth abnormalities, called 'growth arrestment phenomenon' (GAP), which occur extensively in the Sultana vineyards of the Lower Orange River region, field experiments comprising K fertilisation, scheduled irrigation, crop control and specific plant organ and tissue analyses were conducted. Experimental results showed no response to K fertilisation but a promising response to controlled irrigation. Analyses of various organs revealed a faulty carbohydrate metabolism and high N concentrations, probably a result of the fertile soil and high temperatures during the growing season, causing vigorous as well as continued shoot growth after harvest. Specific analyses of affected organs and tissues showed black bunches and shiny spots on leaves to be high in N and Mn and low in K. It is postulated that amongst the major factors causing GAP are the specific climatic and soil conditions which cause poor accumulation of reserves and high N contents. The mechanism by which GAP is caused and practical measures to combat this phenomenon merit urgent further studies. The environmental conditions, the occurence of the growth arrestment phenomenon (GAP) amongst the Vi tis vinifera L. cv. Sultanina vineyards of the Lower Orange River region, the symptons associated with GAP and the analytical characteristics of affected plant organs and associated soil, have already been reported (Saayman, 1983). Preliminary results indicated that K nutrition and irrigation practices may be factors contributing to GAP. The phenomenon of vines having a very high yield in one year, followed by a very low yield as well as sparse budding and retarded growth the following year, led to investigations into the possible contribution of overcropping to the occurrence of GAP, and the efficiency of reserve hydrolyses in spring or the general adequacy of reserves. MATERIALS AND METHODS Two cooperative field experiments were initiated. In Experiment I the following treatments, which were replicated in GAP classified vineyards on eight farms, were evaluated: A: Control, no treatment B: Production control-15 bunches per vine C: K fertilisation-3,0 t K 2 SO/ha plus five 0,5% K 2 S0 4 weekly foliar sprays between bud burst and flowering, followed by fortnightly 3% K 2 S0 4 sprays after harvest until leaf drop D: K fertilisation and foliar sprays as in C, plus production control as in B. Potassium fertilisation was done by injecting saturated K2S04 solution into the soil by means of fumigant pumps in a circular pattern with a radius of c. 1,5 m around each vine. Treatments commenced during 1973 and each plot consisted of four Sultana vines with a known history of GAP. Plots were randomly selected in existing vineyards, and those which received K fertili-sation were situated at least 5 m from other plots. In the case of four farms (replicates), irrigation schedules based on the water holding capacity of the soil, class A-pan evaporation data, a 0,6 crop factor and a 50% available moisture depletion of the soil, amounting to c. 12 flood irrigations of c. 150 mm each during the growing season (Sept.-March), were prescribed. The remaining four producers continued their normal irrigation practices which were generally between 7 and 8 irrigations of 150-250 mm each during the growing season. Soil samples were taken after the application of K, and samples of bearer canes, new shoot growth and leaves were taken at flowering stage and analysed for the major elements using standard VORI methods. Shoot and crop mass were determined during the initial three seasons, but discontinued because of unsatisfactory control over irrigation applications, the lack of success in regulating production and dwindling producers' interest. A final evaluation of GAP affliction was done during October, 1979. Parallel to this investigation, a second trial, Exp. II, was conducted in which eight-vine plots which were monitored during the general survey (Saayman, 1983) were selected in order to have ten plots representing GAP vineyards and another ten plots of normal vines. These plots were sampled as follows: Dormant shoots sampled at pruning Canes sampled just before bud burst Canes sampled just after bud burst Canes sampled during the growth arrestment stage (October). These samples were analysed for starch, sugar and arginine, using standard VORI procedures. In addition to these two experiments, specific case studies were also carried out where good examples of pronounced GAP symptoms were encountered, and plant material were sampled for macro-and micro element analyses. RESULTS AND DISCUSSION K-fertilisation and irrigation effects: The effect of intensive K fertilisation on the soil as investigated in Experiment I, is shown in Table I. It is clear that the massive S. Afr.