The absence of red color in a streaked “Bleaching” pattern is periodically noted on late-season peaches in middle Georgia. The streaked pattern led to a hypothesis that accumulation of pesticides in the stem end of the fruit prevented anthocyanin formation. However, analysis of pesticide residues on affected and unaffected peel suggested this was unlikely. We observed that trees affected by fungal gummosis (caused by Botrysphearia dithodia) were most often affected by the “bleaching” phenomenon and that `Summer Gold', the most fungal gummosis–susceptible variety, had the greatest incidence of the disorder. In a preliminary trial, we tested the hypothesis that fungal gummosis mediates “bleaching” by interfering with anthocyanin color formation in the peel of developing fruit. Tree gum/resin and pesticides were tested for their effect on peel color development. The gum/chemical preparations were dripped onto fruit prior to anthocyanin or red pigment formation in peach peel. After the anthocyanescent period, fruit were observed for bleaching. The gum mediated a negative effect by sulfur, captan, and carbaryl in peel color formation in peach. Fenbuconizole and phosmet had a less negative effect on color formation, although the effect was noticeable. The gum alone, propiconizole, and chlorothalonil did little to effect on peel color formation.
For peach fruit to withstand the rather vigorous postharvest handling during the packing and shipping process, growers pick them fairly early when they are very firm. Yet they soften very quickly, giving them a shelf life of generally 5–15 days. It may be desirable to allow fruit to remain on the tree longer, allowing them to increase in size, while improving in flavor and color. The fact that aminoethoxyvinyl glycine (AVG), ReTain™, interferes with ethylene synthesis was borne out by findings that AVG applications prior to harvest delayed ripening, increased fruit removal force in apple, and reduced preharvest drop. Seven-day preharvest application of 50 g AVG/100 gal to two peach varieties increased firmness from 12% to 54% and moved 7% of the harvest to a date 3–7 days after AVG treatment. The additional time to peak harvest may indicate that the fruit can hang on the tree for a longer period, allowing additional time for fruit expansion. Preliminary indications are that only a 2% to 4% size increase can be achieved. However, this appears to be enough to move 5% to 7% of the fruit into the next ring size.
K.C. Taylor and H.L. Geitzenauer
Macrophylla-decline (MD)-affected citrus display apparent nutrient deficiencies in a sectorial pattern within the citrus tree canopy. The status of several elements (Ca, Cu, Fe, Mg, Mn, and Zn) was assessed in MD and healthy citrus selected from the same citrus orchards. Leaf and phloem tissues were sampled from mature, reproductive trees. Levels of Ca, Cu, Fe, Mg, and Mn were unaffected by the disorder in leaf or phloem tissues. Zinc was diminished in the leaves of MD citrus, and elevated in the whole phloem tissue (2.57-fold on a dry mass basis). Calcium and Cu were sufficient, while Mg, Fe, and Mn were slightly diminished in the leaf tissue, but phloem levels of these elements were not significantly different from that present in the phloem of healthy trees. Since Zn appeared to be redistributed to the phloem tissue from the leaves, the accumulation of the phloem specific, 5-kD Zn-binding protein (ZBP) was assessed in Macrophylla decline trees relative to healthy trees. The 5-kD ZBP was 4.77-fold greater in the phloem of MD citrus relative to healthy. This appears to account for the 2.4-fold greater level of Zn (on a fresh mass basis) found in the crude phloem extracts of the decline-affected citrus relative to healthy. In the purified ZBP fraction from decline-affected citrus, there was 4.73-fold greater Zn than in the ZBP purified from healthy. However, the ratios of Zn to ZBP were equivalent between MD citrus and healthy citrus, suggesting that phloem Zn accumulation in MD citrus is associated with the 5-kD ZBP.
G.C. Wright, W.B. McCloskey, and K.C. Taylor
Four orchard floor management strategies—disking, mowing, chemical mow, and clean culture using herbicides—were evaluated in a `Limoneira 8A Lisbon' lemon orchard in Southern Arizona, starting in the fall of 1993. Disking was the cultural practice used to manage the orchard floor before the start of the experiment. Although disking the orchard floor may have injured shallow tree roots, it provided satisfactory weed control except underneath the tree canopies where bermudagrass, purple nutsedge, and other weed species survived. Chemical mowing with Roundup at 1.168 L/ha did not provide satisfactory control of many weed species and required too many applications to be commercially feasible. This treatment was converted to a combination clean culture and disk treatment (clean and disk) in Summer 1995. Mowing the orchard suppressed broadleaf weed species, allowing the spread and establishment of grasses, primarily bermudagrass, and to a lesser extent, southern sandburr. A fall application of Solicam and Surflan followed by a summer spot treatment application of Roundup was used to control the weed flora in the clean culture treatment. Spot treatment applications of sethoxydim (Poast and Torpedo) were also made to control bermudagrass growing under the tree canopies in the clean culture treatment. Total 1995 yield of the mow, clean & disk, disk, and clean culture treatments were 4867, 5112, 5216, and 6042 kg of fruit, respectively. For the first harvest of 1995, the trees under clean culture also had significantly greater numbers of large fruit than did the trees under the other treatments.
K.C. Taylor, C.D. Chase, L.G. Albrigo, and J. W. Grosser
Citrus blight is an extremely complex decline disorder of unknown etiology, Zinc accumulates in the phloem of the tree 40-50 cm above the bud union 1-3 years prior to visible symptoms of blight (foliage wilt and twig dieback). This is accompanied by Zn deficits in the leaves. A Zn-binding peptide (ZBP) purified from citrus phloem tissue accounts for a symptomatic redistribution of Zn from the canopy to the trunk phloem. ZBP is found in blight and healthy trees and is therefore a normal component of cellular metabolism. To further understand ZBP's role in metabolism two citrus cell culture lines which were selected based on their susceptibility to blight have been characterized as to their growth under Zn treatments as well as Cu and Cd. In addition, their complement of metal-binding constituents is being determined.