Apple rootstocks

Propagation and Rootstocks

   The apple plantations are raised on seedling rootstocks. The use of clonal rootstocks has not been commercialized despite the established superiority for raising uniform plantations, precocity and high productivity in Himachal Pradesh, Jammu and Kashmir and Uttar Pradesh. Non-availability of clonal material in the absence of commercial mass propagation techniques has remained the major constraint. Apples are propagated on seedlings of crab apple or self-pollinizing varieties, Golden Delicious, McIntosh, Yellows Newton and Northern Spy having good seed viability, germinability and seedling growth. The seeds of commercial varieties from fruit juice canning units are also used by nurserymen for raising seedling stocks. However, the seedling stocks lack uniformity in tree size and productivity but show better adaptability to sloppy and shallow soils under rainfed conditions.

Seedling rootstock

          Apple seeds need stratification in moist sand at 4°–7°C for 60–90 days. The water-soaked seeds are placed between 2 and 3cm thick layers of moist sand in wooden boxes or polythene bags during December. The stratification boxes or bags are placed in cool place where the required chilling temperature of less than 7°C for 1,000–1,500hr is met in 60–90 days. The sand is kept moist during stratification. The stratification can be accomplished in the lower chamber of the refrigerator also. The stratification requirement is also met with in areas having very cool winters by direct sowing of seeds in nursery beds during November–December. The pre-stratified seeds, as indicated by whitish tip at the micropylar end, are sown during February–March in well-developed, raised beds. Sowing is done in rows 10cm apart with the spacing of 5cm between the seeds and 2–3cm deep. Nursery beds should be irrigated after sowing and covering with 10cm hay or pine needle mulch to protect the germinating seedlings from heavy rains and spring frost. Mulch when the seedlings are 5cm long for the straight upright growth of seedlings. One-year-old seedling stocks are ready for grafting during February–March.

Clonal rootstocks

           The size controlling Malling (M) and Malling Merton (MM) series clonal rootstocks were introduced at the Regional Horticultural Research Station, Mashobra and Temperate Fruit Research Station, Kotkhai, in Himachal Pradesh, Government Hill Fruit Research Station, Chaubattia in Uttar Pradesh and Fruit Research Station, Shalimar in Jammu and Kashmir during late sixties.

The size-controlling clonal rootstocks suitable for Indian conditions are listed in Table 3.

Table 3. Clonal rootstocks for apple

Category	Rootstock	Characteristics
Dwarfing	M 9	Short juvenile phase, weak anchorage, suitable for high-density planting in flat and irrigated areas only
Semi-dwarf	M4, M7 and MM 106	Suitable for high-density planting and well­drained soils; resistant to wooly apple aphid but susceptible to collar rot
Semi-vigorous	MM 111	Tree size is 70% of standard, drought- tolerant and resistant to wooly apple aphid
Vigorous	Merton 793	Wooly apple aphid and collar-rot resistant, early-fruiting, recommended for Kumaon hills of Uttar Pradesh

          The common method of propagation of clonal rootstocks of apple is mound layering (stooling). The rooted layers of the clone are planted in well-prepared stool beds during winter at a distance of 30cm in the row and 60cm apart. The 3–4 years old layers give rise to numerous suckers during spring. The suckers are covered with the soil before monsoon. The suckers are ringed or notched near the base during the rainy season and covered with soil to encourage rooting. The difficult-to-propagate rootstocks like M 9 need the treatment of 1,000–2,000ppm IBA at the notched portion for quick root initiation. The rooted layers are separated at the onset of dormancy (December) and lined out in nursery beds for further grafting with scion varieties during February–March. The rootstock should be healthy and disease-free and should attain the thickness of 0.9–1.25cm at grafting height for proper bud-take success. The growth of the rootstock should be straight and upright with proper root development. There should be no gall, knot or injury scar at the grafting height.

          The rootstocks of apple are grafted with desirable scion variety during February–March. The scion wood should be collected from the mother plants of known pedigree. It should be collected from bearing trees only. One-year-old shoot growth is ideal for scion wood. Scion sticks should have only vegetative buds and not the reproductive buds. The scion wood should be healthy and disease-free. They should have 3–5 well-developed buds with smooth internodes. These should be collected during dormancy.

            The scion wood collected prior to grafting must be properly stored. It should be kept slightly moist and at a low temperature to prevent the bud-break. A common method is to wrap the wood, in bundles of 25–100 sticks, in polythene sheets or bags. Moist saw-dust, wood shavings or moss should be used for packing to maintain moisture during storage. Sand should be avoided as it sticks to the scion wood and blunts the edge of knife during grafting. The storage temperature is also very important. If the scion wood is stored for 2–3 weeks, 5°C storage temperature is satisfactory. However, if the scions are to be stored for a longer duration (1–3 months), the scion wood should be kept at about 0°C to keep the buds dormant. Scions cannot be used for grafting in the active stage of growth.

               Tongue grafting is the ideal method of grafting scion cultivar on the rootstock with more than 90% bud-take success. The scion should be grafted 15–20cm above the ground level. February–March is most suitable time of tongue grafting. It should be just before the bud-break. In tongue grafting, a slant cut is made on the rootstock 2–3cm long across the stem about 15–20cm above the crown. A vertical cut is given from one-third of the top of slant cut and extended up to two-thirds of the length of slant cut. Similar cuts are made on the scion wood and the 2 parts are connected by inner locking the tongues made by vertical cuts on scion and stock. The cambium of the 2 portions should match for successful graft. The graft union is wrapped with polythene strip. The bud take is accomplished in 4–6 weeks and thereafter the polythene wrapping should be removed.

‘T’ budding during monsoon and chip budding during August can also be done for propagating scions with good bud-take success and smooth scion-stock union but the plant growth is poor in the Indian conditions. It takes one year for raising grafted plants, whereas two in case of budded plants of standard size.