The Degree of Susceptibility of Nectary-Inoculated Cotton Flowers and Bolls to Subsequent Seed Infection by Aspergillus flavus Is Determined at or before Anthesis

In greenhouse and field studies, cotton (Gossypium hirsutum L.) flowers were inoculated with Aspergillus flavus at the involucral nectaries. Bolls developing from early-season flowers had significantly higher percentages of A. flavus-infected seed than did bolls from flowers formed later in the season. Seeds from bolls inoculated 2 weeks after anthesis had the same infection levels as those from flowers inoculated at anthesis. These results indicate that early-season flowers are predisposed to A. flavus infection and that the degree of susceptibility at anthesis is retained through early boll development.     

Presence of Aspergillus flavus in developing cotton bolls and its relation to contamination of mature seeds.

Aspergillus flavus spores were dusted onto the involucral nectaries of cotton flowers. The fungus was present in 20 to 58% of the immature bolls harvested 25 or 35 days after anthesis. Among similarly inoculated bolls fully matured either in the field or under sterile conditions at ambient temperatures after excision from the plants, only 3 to 14% contained A. flavus in the seeds. There was no significant difference in the numbers of contaminated bolls between the excised and field-matured treatments. It is concluded that A. flavus is present in developing cotton bolls before dehiscence, but its presence does not ensure infection of mature seeds, and that excision does not reduce A. flavus contamination if the bolls are maintained at ambient temperatures.     

Presence of Aspergillus flavus in developing cotton bolls and its relation to contamination of mature seeds

Aspergillus flavus spores were dusted onto the involucral nectaries of cotton flowers. The fungus was present in 20 to 58% of the immature bolls harvested 25 or 35 days after anthesis. Among similarly inoculated bolls fully matured either in the field or under sterile conditions at ambient temperatures after excision from the plants, only 3 to 14% contained A. flavus in the seeds. There was no significant difference in the numbers of contaminated bolls between the excised and field-matured treatments. It is concluded that A. flavus is present in developing cotton bolls before dehiscence, but its presence does not ensure infection of mature seeds, and that excision does not reduce A. flavus contamination if the bolls are maintained at ambient temperatures.     

Nectaries as Entry Sites for Aspergillus flavus in Developing Cotton Bolls

Cotton flowers in replicate plots in two fields near Phoenix, Ariz., were tagged in June at the beginning of the flowering period. Flowers or developing bolls from these tagged flowers were inoculated on the involucral (bracteal) nectaries with dry spores of Aspergillus flavus. The bolls were harvested as they matured in August, and the seeds were assessed for the presence of the fungus. The number of infected seed from flowers and bolls inoculated up to 25 days after anthesis was significantly higher than that in uninoculated controls. Seeds from bolls inoculated after 25 days postanthesis did not differ significantly from controls in degree of infection. We postulate that the sharp decline in the ability of the fungus to infect the plant and seed is a result of physical or biochemical changes in the boll as it reaches physiological maturity or biochemical changes in the entire plant as it develops.     

The occurrence of Aspergillus flavus in vegetative tissue of cotton plants and its relation to seed infection

Twenty-seven mature cotton bolls with Aspergillus flavus Link colonies naturally occurring on the surface of the boll or lint were collected in the field in Arizona along with their subtending stems and peduncles. Bolls inoculated through the carpel wall 30 days after anthesis were allowed to mature in the field and were collected in the same manner. The seed and stem and peduncle sections of each boll were surface-sterilized, plated on agar media and observed for A. flavus. Seventy-eight percent of the naturally contaminated bolls with A. flavus in the seed also had the fungus in the stem and peduncle, whereas only 31% of the naturally contaminated bolls with no A. flavus in the seed had the fungus in the stem or peduncle. This difference was significant (P=0.0125), indicating a positive relationship between seed infection and stem and peduncle infection. All of the bolls inoculated through the carpel wall had A. flavus in the seed, but only 11% of the stem and peduncle sections were infected, indicating that the fungus does not readily grow downward from the boll into the supporting stem or peduncle.This unidirectional pattern of movement (upward) was further substantiated in greenhouse experiments where cotton seedlings were inoculated at the cotyledonary leaf scar with A. flavus and plants were sequentially harvested, surface sterilized and plated. Aspergillus flavus was isolated from the cotyledonary leaf scar, flower buds, developing bolls, and stem sections in the upper portion of the plant. It was never isolated from roots or stem sections below the cotyledonary node, again indicating that the fungus does not readily move downward through the plant.     

Bacterial Antagonists of Aspergillus flavus

In order to search for bacteria capable of reducing the aflatoxin contamination of cottonseed, 892 indigenous bacterial isolates, including 11 that were endophytic to cotton, were screened for their ability to inhibit the growth of Aspergillus flavus on cottonseed in an in vitro bioassay. Only six isolates partially or totally inhibited fungal growth. All antagonistic isolates were recovered from boll, lint or seed surface or from the lint of mature bolls. One was retrieved from mature seeds. None of the endophytic isolates showed activity. In four field trials, the incidence of A. flavus -induced damage to locules inoculated simulteously with A. flavus plus the most A. flavus plus the most effective antagonistic isolate (D1) was reduced by 41-100% relative to locules inoculated with A. flavus alone. The severity of damage to locules inoculated simultaneously with A. flavus and with D1 was reduced by 60-l00% relative to locules inoculated with A. flavus alone. Isolate D1, identified as Pseudomonas cepacia, completely inhibited the growth of A. flavus on synthetic media.     

Conservation implications of the reproductive ecology of Agalinis acuta (Scrophulariaceae)

Reproductive ecology of Agalinis acuta was investigated by examining potential for self-fertilization before and at anthesis, reproductive output from outcrossed vs. selfed matings, and effects of browsing, plant size, and conspecific plant density on seed and fruit production. These features of a plant species can provide indirect information pertinent to conservation such as patterns and maintenance of genetic diversity, risk associated with inbreeding depression, and changes in pollinator abundance or effectiveness. The species is self-compatible, with 97% of selfed flowers setting fruit; pollinators were not required for reproduction. However, seed set in self-pollinated fruits averaged 17-20% less than that in open-pollinated fruits. Geitonogamous and facilitated selfing are possible throughout anthesis and autonomous selfing is possible late in anthesis as corollas abscise. Delaying self-pollination until after outcrossing opportunities likely limits selfing rates and thus reduces risks associated with inbreeding but allows reproduction in absence of pollinators. Supplementing pollen on open-pollinated flowers yielded no additional seed set over controls. Neither early-season browsing of primary stems nor conspecific plant density had significant effects on number of fruits per plant, on fruit size, or on number of seeds from open-pollinated flowers. Currently, reproduction appears to be high (about 2400 seeds/plant), and future risks due to lack of genetic diversity are likely low.     

Cotton boll abscission and yield losses associated with first-instar bollworm (Lepidoptera: Noctuidae) injury to nontransgenic and transgenic Bt cotton

Field tests were conducted in northeastern Louisiana to determine the effects of infestations by Helicoverpa zea (Boddie) on cotton bolls of varying ages. First instars were caged on bolls of nontransgenic ('Deltapine 5415') or transgenic Bacillus thuringiensis Berliner variety kurstaki (Bt) ('NuCOTN 33B') cotton from 29 June to 11 August during 1997 and 1998. Deltapine 5415 bolls that accumulated 179 (7.2 d), 281 (11.2 d), and 253 (10.1 d) heat units beyond anthesis were safe from bollworm-induced abscission at 72 h after infestation, 7 d after infestation, and at the time of harvest, respectively. NuCOTN 33B bolls that accumulated 157 (6.3 d), 185 (7.4 d), and 180 (7.2 d) heat units beyond anthesis were safe from bollworm-induced abscission at 72 h after infestation, 7 d after infestation, and at the time of harvest, respectively. Bollworm larvae reduced seedcotton weights of Deltapine 5415 bolls that accumulated between 58.5 (2.3 d) and 350.5 (14.0 d) heat units beyond anthesis. Seedcotton weights of NuCOTN 33B bolls that accumulated between 0 and 281 (11.2 d) heat units beyond anthesis were reduced by bollworm injury. Deltapine 5415 and NuCOTN 33B bolls that accumulated 426.5 (17.1 d) and 299.5 (12.0 d) heat units beyond anthesis, respectively, before infestation were not injured by first-instar bollworm larvae. These data provide information about late-season insecticide termination strategies for bollworms on nontransgenic and transgenic Bt-cotton. This, in turn, will help pest managers determine when insecticides are no longer economical during the late season.     

Transmission of Alternaria macrospora in Cotton Seeds

Alternaria macrospora was isolated from seeds only after the natural opening of the bolls and exposure of seeds to an environment in which the fungus was present. The fungus lacks the ability to penetrate the boll wall and reach the seed site. Attempts to isolate the pathogen from seeds of immature bolls at different developmental stages failed. Internal infection by slow injection resulted in seed infection and partial shedding of the injected plant parts which was high in buds and decreased with the ripening to mature bolls. Severity of seed infection was not dependent on either inoculum level, boll physiological age or even if the boll itself was not diseased. Infection of flowers under field conditions caused flower shedding. Naturally infected seeds or inoculated seeds with inoculum levels of 100 spores/ml and above resulted in diseased cotyledons, the incidence of which was, for inoculated seeds, positively correlated with inoculum level. A small difference was observed between cultivars in susceptibility to artificial inoculation at the cotyledon stage. A. macrospora survived on commercial cotton seeds and on post-season plants left growing at the field edges. Survival in plant debris under field conditions was minimal and may only have a minor effect on field reinfestation.     

Variability among atoxigenic Aspergillus flavus strains in ability to prevent aflatoxin contamination and production of aflatoxin biosynthetic pathway enzymes.

Five strains of Aspergillus flavus lacking the ability to produce aflatoxins were examined in greenhouse tests for the ability to prevent a toxigenic strain from contaminating developing cottonseed with aflatoxins. All atoxigenic strains reduced contamination when inoculated into developing bolls 24 h prior to the toxigenic strain. However, only one strain, AF36, was highly effective when inoculated simultaneously with the toxigenic strain. All five strains were able to inhibit aflatoxin production by the toxigenic strain in liquid fermentation. Thus, in vitro activity did not predict the ability of an atoxigenic strain to prevent contamination of developing bolls. Therefore, strain selection for competitive exclusion to prevent aflatoxin contamination should include evaluation of efficacy in developing crops prior to field release. Atoxigenic strains were also characterized by the ability to convert several aflatoxin precursors into aflatoxin B1. Four atoxigenic strains failed to convert any of the aflatoxin biosynthetic precursors to aflatoxins. However, the strain (AF36) most effective in preventing aflatoxin contamination in developing bolls converted all tested precursors into aflatoxin B1, indicating that this strain made enzymes in the aflatoxin biosynthetic pathway.     

Response of cotton to prebloom square loss

In 1996 and 1997, various intensities of prebloom square removal were applied to three cultivars of cotton grown in Mississippi. With the exception of one cultivar in 1997, all cultivars were B. thuringiensis (Bt)-transgenic cotton. At harvest, the number of bolls and seed cotton weight was recorded for all plants in each square removal treatment. All cultivars responded similarly to square loss. A yield increase (overcompensation) was observed in the treatment where all squares were removed from the plant one week after squaring began. Only the treatment where all squares were removed before bloom significantly reduced yield and caused a large (>7 d) delay in crop maturation. Otherwise, moderate levels of square removal (approximately 20-50% of prebloom squares) had little impact on overall lint production. However, the patterns of cotton production on the plants were significantly influenced by the square removal treatments. The removal of relatively more or larger squares increased seed cotton production in late-season fruiting cohorts and on 'vegetative' branches. Compensation for square loss occurred by increasing the relative number and weight of bolls produced subsequent to early-season square removal. Typically, early-season square loss increased the value of later-season fruiting cohorts, especially the midseason cohorts and bolls on vegetative branches. The implications of prebloom square loss, including the compensatory ability of the cotton plant, on insect management are discussed.     

Injury to preflowering and flowering cotton by brown stink bug and southern green stink bug

The impact of brown stink bug, Euschistus servus (Say), and southern green stink bug, Nezara viridula (L.), injury was evaluated on preflowering and flowering cotton, Gossypium hirsutum L., plants in no-choice tests. Vegetative stage cotton seedlings and reproductive structures, including flower buds (square) and bolls, were infested with adults and/or nymphs of both species. There were no significant differences in height, height to node ratio, square retention, and flower initiation for cotton seedlings or plants with a match-head square between southern green stink bug adult- or brown stink bug adult-infested and noninfested treatments. Abscission for individual large squares (precandle) and multiple squares (medium and small square on the same sympodial branch) was not significantly different among infested and noninfested treatments for the following species and developmental stages: brown stink bug adults, southern green stink bug adults, and third and fourth to fifth instar southern green stink bug nymphs. In boll infestation studies, the relationship between boll maturity, expressed as heat units beyond anthesis, and boll growth, abscission, hard locked carpels, seedcotton yield, and seed germination was measured. Brown stink bug induced abscission in bolls that had accumulated > 0-350 heat units beyond anthesis. Boll growth and seedcotton yield was significantly lower for bolls infested with brown stink bug through 266.5 and 550 heat units beyond anthesis, respectively. The proportion of hard locked carpels per boll was significantly greater for the infested treatment in a cohort of bolls that accumulated from 51 to 400 heat units beyond anthesis. Seed germination in bolls infested with brown stink bug was significantly lower in bolls aged 101-600 heat units beyond anthesis.     

Influence of Nezara viridula feeding on cotton yield, fiber quality, and seed germination

The influence of southern green stink bug, Nezara viridula (L.), adults (males and females) and fourth to fifth instars on cotton, Gossypium hirsutum L., boll abscission, seedcotton yield, fiber quality, and seed viability was evaluated in field studies conducted during 2004 and 2005. Cotton bolls representing several age classes ranging from 0-600 heat units were individually infested with a specific gender or life stage of southern green stink bug. Adults and nymphs induced abscission of bolls that accumulated 0-280 heat units after anthesis. Seedcotton yield was significantly lower in bolls infested with adults (males and females) and late instars through approximately 500 heat units after anthesis. Southern green stink bug feeding on bolls significantly affected the physical fiber properties of micronaire (measure of fiber fineness or maturity), strength, uniformity, and fiber length. Discolored cotton lint in the stink bug-infested bolls was more common than in noninfested bolls. Seed germination and development of normal seedlings for seed harvested from stink bug-infested bolls that accumulated < or =500 heat units beyond anthesis were significantly lower compared with noninfested bolls. No significant differences in boll abscission, yield, fiber quality, and seed germination were detected between southern green stink bug males and females or between adults and fourth to fifth instars.     

The roles of abscisic acid and ethylene in the abscission and senescence of cocoa flowers

Cocoa flowers have a limited period of longevity; more than 90% of unpollinated flowers abscised within 32 h after anthesis. Abscisic acid (ABA) levels increased significantly prior to abscission. By 21 h after anthesis, ABA levels had increased almost 10-fold, and by 32 h flowers had 20-fold higher levels of ABA than at anthesis. Fluridone completely inhibited both the increase in ABA, the formation of an abscission zone, and the abscission and senescence of flowers. In contrast, ethylene production increased only slightly 21 h after anthesis and was only 2-fold higher after 32 h. Aminoethoxyvinylglycine (AVG) delayed but did not prevent abscission. In cocoa flowers, ABA is the primary regulator of abscission; ethylene accelerates abscission but only in the presence of ABA. Naphthalene acetic acid (NAA) treatment of flowers at anthesis prevented abscission zone formation and flower abscission, but did not induce fruit set. All parts of the NAA-treated flower except the pedicel senesced after 6 days. NAA+AVG treatment only delayed, whereas fluridone treatment completely prevented flower senescence.     

Changes in Amide-Linked and Ester Indole-3-Acetic Acid in Cotton Fruiting Forms during Their Development

The concentration of free indoleacetic acid (IAA) is high in cotton (Gossypium hirsutum L.) fruiting forms before anthesis, but is low at and for a few days after anthesis. Amide-linked and ester IAA were measured in fruiting forms at 9, 6, and 3 days before anthesis; at anthesis; and at 2, 4, 7, and 9 days after anthesis to determine if free IAA decreased because it was converted to a conjugated form. That did not appear to be the case. While the major decrease in free IAA occurred during the 6 days before anthesis, ester IAA increased only a small amount and amide-linked IAA decreased even more than free IAA. During the 6 days before anthesis free IAA decreased from 0.62 to 0.12 micrograms per gram and amide-linked IAA decreased from 19.14 to 1.16 micrograms per gram dry weight. No evidence was found that a large amount of amide-linked IAA was converted to an insoluble form; flowers contained less than 1 microgram per gram of insoluble IAA. The free and amide-linked IAA must have been converted to other forms, perhaps by oxidation. Soluble amide-linked IAA remained low after anthesis. No ester IAA was detected 6 days before anthesis and only 0.08 microgram per gram dry weight was measured at anthesis. The concentration of ester IAA increased thereafter to 4.43 micrograms per gram at 9 days after anthesis. Therefore, amide-linked IAA was the major form of IAA in flower buds and ester IAA was the major form in young fruits (bolls). Minimum concentrations of free and total IAA occurred during the 4 days after anthesis, a stage when cotton fruiting forms are most likely to abscise. The large decreases in free and amide-linked IAA during the 6 days before anthesis may indicate a rapid turnover of IAA in flower buds. But, the decrease in free IAA was not accompanied by a comparable increase in ester or amide-linked IAA.     

Short-term effects of burn season on flowering phenology of savanna plants

We examined the effect of season of burn on flowering phenology of groundlayer species, in the year following burns, in a mesic-sand Midwestern oak savanna. Burn treatments were fall, early-season, growing-season, late-season, and 1 or 5 years after a prior early-season wildfire. For these treatments, we compared the number of flowering stems and of flowers for species overall, for the 20 most prolifically flowering species, as well as for species grouped by flowering phenoperiods, and by growth form. Growing-season burn had a significant negative effect on number of flowering stems and total number of flowers. This effect occurred when either the burn occurred during the flowering season or during the season prior to the flowering phenoperiod. Tradescantia ohiensis showed expedited flowering and Phlox pilosa showed delayed flowering in response to early-season burning. Flowering of early shrubs was reduced by the previous fall and early-spring fires, while flowering of mid-season blooming shrubs was reduced by the early- and growing-season burns. Vaccinium and Gaylussacia, early-flowering shrubs, produced fewer flowers 1 year after than 5 years after an early-season burn. Arabis lyrata showed reduced flowering from the early-season burn. We also found four instances where the early-spring burn effect on flowering was more severe than the fall burn effect, suggesting that many frequent early-season burns may be deleterious to flowering and reproduction of some species. Burns occurring too frequently in the same season could negatively affect future flowering and reproduction of these plant species.     

Fruit Abscission in Cowpea, Vigna unguiculata (L.) Walp.: II. QUANTITATIVE ESTIMATIONS OF TOTAL NUCLEIC ACIDS IN OVARIES OF PERSISTING AND ABSCISSING FLOWERS

The fresh weights and quantities of total nucleic acids (RNAand DNA) in the ovaries of abscissing flowers and persistingflowers were estimated at intervals. Starting from 48 h prior to anthesis, particularly during the24 h immediately preceding it, large differences were observed:ovaries of abscissing flowers were much lighter and accumulatedmuch less nucleic acid than those of persisting flowers. Forinstance, at anthesis the ovaries of persisting flowers (atraceme 1, in acropetal order) were twice as heavy, and containedtwice as much DNA and one-and-a-half times as much RNA as theovaries of abscissing flowers (at raceme 3). These differenceswere further maintained from after the flowers had opened untileventual shedding of the raceme 3 flowers. The results provided clear evidence of the very basic retardationexperienced by the abscissing flower before and after opening.     

外源乙烯利施用时期对花生源库形成的调控效应

为了解决源库关系不协调而限制花生产量提高的问题,在大田栽培条件下,以‘山花9号’花生为试验材料,设置花后10、20、30 d 3个喷施时期,以不喷施处理为对照,探讨不同时期喷施乙烯利对花生源库形成的调控效应。结果表明: 花后10和20 d喷施乙烯利可显著减少花生的开花数量、果针数、幼果数,提高秕果数和饱果数,而花后30 d喷施处理对开花数量、果针数和幼果数无抑制作用。喷施乙烯利可以增加花生单株叶面积,开花后10 d喷施处理的单株叶面积增幅最大,随着喷施时期的推迟增幅减小。花后10和20 d喷施乙烯利显著提高了花生叶片的光合速率,但花后30 d喷施处理只能在短期内提高光合速率,对生育后期的叶片光合速率无显著影响。从源库综合性状来看,花后20 d喷施乙烯利的源库关系最协调,有利于促进同化物向荚果的运输,提高有效果比例和荚果充实度,从而提高产量。因此,喷施乙烯利是解决花生“花多不实、果多不饱”源库失衡现象的有效措施,生产中使用乙烯利控花应选择在开花后20 d喷施。     

Cotton boll age influences feeding preference by brown stink bug (Heteroptera: Pentatomidae)

Cotton plants were infested with brown stink bug, Euschistus servus (Say), to define cotton boll age classes (based on heat unit accumulation beyond anthesis) that are most frequently injured during each of the initial 5 wk of flowering. Bolls from each week were grouped into discrete age classes and evaluated for the presence of stink bug injury. Brown stink bug injured significantly more bolls of age class B (approximately 165-336 heat units), age class C (approximately 330-504 heat units), and age class D (approximately 495-672 heat units) during the initial 3 wk in both years and in week 5 in 2002 compared with other boll ages. Generally, the frequency of injured bolls was lowest in age class A (< or = 168 heat units) during these periods. The preference by brown stink bug for boll age classes B, C, and D within a week was similar when ages were combined across all 5 wk. Based on these data, bolls that have accumulated 165.2 through 672 heat units beyond anthesis (approximately 7-27-d-old) are more frequently injured by brown stink bug when a range of boll ages are available. The boll ages in our studies corresponded to a boll diameter of 1.161-3.586 cm with a mid-range of 2.375 cm. A general protocol for initiating treatments against stink bugs is to sample bolls for evidence of injury as an indicator of presence of infestations in cotton. Sampling bolls within a defined range, which is most likely to be injured, should improve the precision of this method in detecting economic stink bug infestations in cotton.     

The effect of inoculating flowers and developing fruits with Botrytis cinerea on post-harvest grey mould of red raspberry

Raspberry flowers were inoculated in the glasshouse and field with dry conidia of Botrytis cinerea and the fruits derived from them subjected to post-harvest rot tests at c. 20°C and high humidity. Apparently healthy fully-ripe picked fruits derived from inoculated flowers developed grey mould faster than those from non-inoculated flowers in all tests. In the glasshouse experiments, fruits from inoculated tightly closed flower buds rotted more slowly than those from inoculated open flowers or those at later developmental stages. Fruits from inoculated whole flowers rotted more rapidly than those from emasculated flowers; the addition of pollen to emasculated flowers had little effect on post-harvest grey mould. In the dry summer of 1984 no fruits in the field from inoculated whole flowers rotted before ripening, but in the wet season of 1985 pre-harvest grey mould was common and the surviving healthy fruits rotted in c. 1 day after picking. Only minor differences were detected in host susceptibility to post-harvest grey mould in both glasshouse and field tests, the ranking of genotypes varied depending on whether or not flowers had been inoculated. The susceptibility of pistils of 40 Rubus genotypes to infection was examined 7 and 28 days after inoculation of stigmas with dry conidia. Conidia germinated on the stigmas and produced hyphae which grew through transmitting tissues of the styles to infect carpels symptomlessly in 17 red raspberries, one blackberry, two Rubus spp. and one hybrid. No germination occurred on stigmas of cv. Carnival and New York Selection 817.