Determining the Chill Requirement of important Stone Fruit Rootstocks available to the South African Fruit Industry

The dormancy progression and chill requirements of stone fruit rootstocks were assessed to better understand their depth and patterns of dormancy. This information is vital when choosing rootstock /scion combinations best suited to the low chill and varied growing conditions prevalent in South Africa.

Fifty nine dormancy progressions and 15 chill requirement (CR) calculations were generated over 2 years from 2 different locations using standard forcing laboratory procedures. A 2 line fit model was fitted to each of the dormancy progressions and nine variables identified from the estimated regression parameters were used in a discriminant analysis (DA). The discriminant plot factors from the DA were then clustered using Ward’s Agglomerative hierarchical clustering (AHC). Univariate clustering was also performed on the means of the individual variables (criteria) for each rootstock. The 2 locations were analysed separately.

Dormancy progressions varied tremendously between rootstocks. Rootstocks entered and exited dormancy over an extended period of time and the depth of dormancy varied from 26 to 112 days. Three distinct dormancy pattern were identified viz. rapid entrance/slow exit, slow entrance/rapid exit and equal entrance and exit.

The different dormancy patterns were used to distinguish between high and low chill rootstocks. High chillers start dormancy early, go into deep dormancy and exit dormancy late. The opposite is true of low chillers. The DA, AHC and Univariate clustering were also used to cluster rootstocks into high and low chillers. Where rootstocks were present at both locations, they always fell into the same cluster pattern. Maridon, SAPO778, Viking, GF677, Atlas, Tetra, Garnem, Cadaman, Kuban86, Krymsk6, Krymsk7 and Tsukuba clustered into the higher chill group while Kakamas, Marianna, Flordaguard, Felinem, Monegro, Chuche Picudo, Guardian and Penta clustered into the lower group. Many of the rootstocks which clustered into early exit from dormancy did not cluster into the lowest maximum dormancy cluster. Caution must be observed when categorising a variety as a high or low chiller solely on blossom date.

High chill rootstocks increase the chill requirement of low chill scions, furthermore when exit from dormancy of the rootstock and scion are not synchronised suckering or poor growth and chlorosis occurs.

The majority of calculated CRs seemed rather irrational. Under South African climatic conditions, determination of maximum dormancy for shoot collection proved difficult, furthermore the variability of maximum dormancy between locations and seasons as well as certain methodology constraints prevented accurate CR calculation.

Under conditions of inadequate chill, the method widely used to determine Chill Requirement as an absolute value is neither accurate nor reliable. The dormancy of rootstocks must rather be assessed generally using both Dormancy Progression and Chill Requirement data over numerous locations and seasons before rootstocks are categorised as low or high chill.