Goldspot, gold speck
Description
Gold specks or
flecks are often observed around the calyx and shoulders of mature fruit,
particularly in summer. In green fruit, the specks are white and less
abundant. These specks decrease the attractiveness of the fruit and significantly
shorten its shelf life (Janse, 1988).
Cells with the characteristic gold appearance were identified by Den Outer
and van Veenendaal (1988) as containing
a granular mass of tiny calcium salt crystals, probably calcium oxalate.
Causes
These specks are considered to be symptoms of excess calcium in the fruit.
De Kreij et al. (1992) found that under
conditions of high air humidity and high Ca/K ratios, more calcium was
transported into the fruit and the incidence of gold speck increased.
Increasing the P level increased also increased calcium uptake rate and
increased speckling. As summarized in Ho et
al., (1999), the level of goldspot
was decreased by increasing NO3 or reducing Cl, NH4, K or EC in the feed),
presumably because these reduced the uptake of calcium.
As temperatures increase during the growing season, the incidence of goldspot also increases, particularly when average greenhouse temperatures were higher than usual. Since the amount of calcium in the fruit did not increase, high temperatures may increase the proportions of calcium deposited in the fruit as oxalate (Ho et al.,1999).
Control
The disorder can
be reduced by avoiding susceptible cultivars (Ilker
et al., 1977). Cultivars resistant
to BER tend to be more susceptible to goldspot (Ho
et al., 1999). Sonneveld and Voogt (1990)
found that raising the electrical conductivity of the nutrient solution
reduced gold speck incidence, as did increasing the K/Ca ratio and increasing
Mg. Presumably in all 3 cases, the mechanism was prevention of excess
Ca uptake. Ho et al. (1999) recommended: lowering
Ca in the hydroponic solution from 200 mg l-1 to 120 mg l-1;
lowering fruit temperature; applying low but sufficient N (180 mg l-1);
sufficient, but not too high K (400 mg l-1); and avoiding P
depletion (>5 mg l-1). Following these guidelines, goldspot
can be reduced, while avoiding BER.