Management of heart rots

Prior to the development of modern fungicides, any method of improving soil drainage was used to reduce disease. Raised beds, ranging from a few centimetres to 25 cm or more, have been used. Improvements in surface drainage, whereby depressions are drained by cutting of ditches or filling to eliminate standing water, have reduced disease levels (Pegg, 1969). Cultural practices such as pineapple-trash mulch have generally, but not always, increased disease incidence.

The addition of elemental sulphur to soil decreased heart rot when the soil bacteria Thiobacillus was present to oxidize the sulphur (Pegg, 1977). The addition of sulphur drastically lowers soil pH (below 3.8), which results in reduced sporangial formation and an explosion of Tricoderma sp. Various copper fungicides have been tried for heart-rot control, but, due to the sensitivity of pineapple to elemental copper, have not been used.

Early organic fungicides used for heart-rot control were fenaminosulf (Dexon®), captan and the dithiocarbamates (Pegg, 1969). More recently captafol (Difolatan®), metalaxyl (Ridomil®) and fosetyl aluminium (Allette®) have been used (Rohrbach and Schenck, 1985). Currently, fosetyl aluminium is used very effectively as a preplant dip at rates of 2.24 kg active ingredient (a.i.) 935 l-1 (see Fig. 9.13). Initial control from the pre-plant dip can be extended by foliar applications with rates of 6.72 kg ha-1 in 2805 l of water at intervals of 3-6 months. Because fosetyl aluminium acts systemically in the pineapple plant, excellent control of P. cin-namomi root rot can be obtained (Rohrbach and Schenk, 1985; Rohrbach and Apt, 1986).

The active ingredient of fosetyl aluminium is phosphorous acid, which will result in comparable control to the parent material (Rohrbach and Schenck, 1985). In Australia, a formulation of phosphorous acid (Phos-for-pine® or Fosjet 200®) is used. Because many growers do not have dipping equipment, pre-harvest sprays are used to systemically protect crowns for planting following fruit harvest (D. Bartholomew, personal communication).

The fungicide metalaxyl has also been shown to be very effective for heart-rot control as a preplant 'seed-piece' dip (Rohrbach and Schenck, 1985). Postplant foliar applications of metalaxyl, although effective, have not been recommended or registered because of the possibility of development of resistant strains of Phytophthora.

Several cultivars resistant to Phytophthora were developed at the Pineapple Research Institute of Hawaii (PRI). The cultivar designated '53-323' with resistance to P. cinnamomi was a cross between 'Smooth Cayenne' and F-236. F-236 was an introduced native variety from Columbia called 'Pina de Castilla', which was collected for PRI by Harold St John. The cultivar '53-323' is highly resistant to P. cinnamomi but highly susceptible to P. parasitica. A second cultivar, designated '59-656', is resistant to both P. cinnamomi and P. parasitica. Both cultivars have fruit charac teristics and quality similar to those of 'Smooth Cayenne' and yields at least equal to 'Smooth Cayenne' when that variety is grown without occurrence of heart rot or root rot (Pineapple Research Institute of Hawaii, unpublished results).

Sanitation is an important factor in preventing initial low incidences of bacterial heart rot from causing an epidemic. Infected plants should be destroyed or removed from the field, as they may provide a source for secondary inoculum. Crowns or slips from plants with symptoms of fruit collapse or from an area having high a incidence of fruit collapse should not be used as seed material. Mechanical leaf damage, such as occurs when entering a field for crop logging, should be minimized during periods of susceptibility and when low levels of disease are present (K.G. Rohrbach, personal observation).

Partial control of bacterial heart rot has been obtained with miticides (e.g. endo-sulphan) and insecticides in the Philippines (K.G. Rohrbach, personal observation). Bordeaux mixture has resulted in variable control (Lim, 1985).

In subtropical climates, where the disease is a problem, the resistant 'Smooth Cayenne' cultivar might be used rather than the much more susceptible 'Spanish' types (Lim, 1971). However, in the lowland tropics, 'Smooth Cayenne' is difficult to force and may have poor fruit quality, thus limiting its use.

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