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    A study of the effects of storage environments and of rice weevil (Sitophilus oryzae L.) on seed deterioration in maize (Zea mays) : a thesis presented in partial fulfilment of the requirements for the degree of Master in Agriculture Science (Seed Technology) at Massey University, New Zealand
    (Massey University, 1983) Khan, I
    Seed of maize variety XL45 was adjusted to three different initial moisture contents (12.4, 15.1 and 18.5%) and stored under four different environments (20 C - 40% RH, 20 C - 65% RH, 30 C - 40% RH and 30 C - 65% RH) for 20 weeks. Half of the seed samples were inoculated with adult rice weevils (Sitophilus oryzae). The remaining seed samples were stored free of insects. Sampling was carried out after 2, 4, 6, 8, 12, 16 and 20 weeks storage. Measurements of seed moisture content, germination, storage fungal development and changes in insect population number and survival were made at each sampling time. Assessments of the number and categories of abnormal seedlings present in laboratory germination tests and internal seed damage caused by insects were also carried out. The changes in initial seed moisture content in response to the relative humidity level in the storage environment were rapid. All seed samples reached equilibrium moisture contents within the first four weeks of storage irrespective of initial moisture level. Equilibrium moisture content in all cases was below the 15% considered safe for short term storage of maize. In the absence of insects, and in environments involving a 40% level of humidity, no extensive reduction in germination percentage generally occurred. However, a relatively small drop in germination capacity was observed late in the storage period in the most extreme combination (initial moisture content 18.5%, 30 C, 65% RH storage environment, 16-20 weeks storage). Major deterioration in seed quality occurred only in those storage environments suitable for rice weevil development i.e. 20 C - 65% RH, 30 C - 65% RH. In particular, loss of germination and increase in both the number and categories of abnormal seedlings were apparent in these treatments. X-ray photography of seeds from different storage environments showed the internal damage caused by rice weevils and this was related to seedling development in sand tests and to normal and abnormal seedling production in standard laboratory germination tests. Under favourable conditions (20 C - 65% RH and 30 C - 65% RH) rice weevil numbers increased dramatically. This increase was greatest at 30 C and also resulted in an increase in seed moisture content. The other storage conditions (20 C - 40% RH and 30 C - 40% RH) were unfavourable for insect survival. This was a direct effect of the low level of relative humidity which resulted in the death of adult rice weevils and prevented the development of larval populations. Studies on the rate and extent of internal seed damage using X-ray techniques showed that germination did not deteriorate until larvae had eaten sufficient of the endosperm to prevent adequate food reserves being available for the seedling. The level of damage to seed viability by Sitophilus oryzae was clearly a function of the size of the insect population and the time over which it persisted. The maize seed used in this study was substantially infected by storage fungi initially. However, despite the provision of high temperature (30 C) and a moderate humidity level (65%) for up to 5 months storage fungi did not cause total loss of germinability despite some increase in the levels of Aspergillus spp. and Penicillium spp. Apparently, even the most extreme environments used in this study were relatively unfavourable for storage funga1 development.
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    The association between some Fusarium spp. and seed quality in maize (Zea mays L.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Seed Technology at Massey University, New Zealand
    (Massey University, 1995) Kabeere, Flavia
    The effect of delayed harvest on the occurrence and incidence of seed-borne Fusarium spp. and their effects on seed quality was investigated using four maize cultivars (Pioneer 3551,3591,3709 and 3475) over two seasons (1989/90,1990/91) at Massey University, Palmerston North. As harvest was delayed from April to July, the percentage of cobs showing Fusarium mould increased. Cultivar 3551 tended to develop Fusarium cob mould later in the season (June) than the other three cultivars. In both seasons the percentage of seeds of all four cultivars infected with Fusarium spp. increased as harvest was delayed. However, there was a difference between the two seasons; in 1989/90 the mean percentage of seeds carrying Fusarium spp. was 26%, 39%, 70% and 82% for April, May, June and July harvests respectively, while the corresponding levels for 1990/91 were 1%, 9%, 31% and 40% respectively. Between season differences were ascribed to climatic differences, the former season being wetter and warmer than the latter. There were only minor differences among cultivars for the percentage of seeds carrying Fusarium spp. F. graminearum was the species most consistently detected in all cultivars in both seasons, being recorded from 16%, 31%, 53% and 72% of seeds from the 1989/90 April to July harvests respectively, and from 0%, 6%, 25% and 30% of seeds from the same harvest times in 1990/91. F. subglutinans, F. poae and other Fusarium spp. were also detected, but their incidence was generally low. Seed-borne Fusarium did not significantly reduce seed germination or vigour. In both seasons germination was between 86-99% for all cultivars. However, any dead seeds bore evidence of F. graminearum mycelial growth. Mycotoxins were recorded in seeds from all harvests in both seasons and mycotoxin levels increased as harvest was delayed. However, there were differences between seasons, as mean levels of Zearalenone, αZearalenol, Nivalenol and Deoxynivalenol ranged from 0.06 - 1.42 mg/kg seed in 1989/90, but from 0.0 - 0.54 mg/kg seed in 1990/91. In all cultivars and at most harvests in both years, levels of αZearalenol and of Nivalenol increased earlier than those of Zearalenone and Deoxynivalenol. Mycotoxin differences among cultivars and the precise nature of the relationship between specific Fusarium species and mycotoxin development urgently requires further study, because of the potential for human and animal health problems. Fusarium spp. from seed-culture colony were initially identified macroscopically on Malt Agar (MA), with pure cultures later being verified by the International Mycological Institute (UK). Subsequently, cultures were studied on Potato Dextrose Agar (PDA), Malt Extract Agar (MEA) and on Carnation Leaf Agar (CLA), with the final identity of seed-culture colonies being verified on CLA. Colony texture and colour (including agar pigmentation) were initially used to separate Fusarium species detected on MA from infected seeds after harvest into a series of groups, ie 'red and fluffy', 'red centre', 'red and lobed', 'cream and fluffy', and 'cream and lobed' for F. graminearum. F. crookwellense was also separated as a 'red centre' type of colony while F. culmorum was separated as 'cream and flat', F. subglutinans 'purple and strands' type, and F. poae as 'purple/white/cream and powdery' type. While it was possible to differentiate the five types of F. graminearum on MA, it was not possible to distinguish F. graminearum 'red centre' type from F. crookwellense, although F. culmorum was relatively easy to differentiate from F. graminearum and F. crookwellense. Use of PDA or MEA pure cultures to differentiate F. graminearum from F. crookwellense or F. culmorum was not successful because the colony morphology of these three species was similar. However, F. subglutinans and F. poae were readily identified macroscopically on MA and MEA. F. graminearum seed-culture colonies which did not sporulate on MA or MEA in most cases readily formed perithecia of Gibberella zeae on CLA (in the presence of 40W NUV light) regardless of whether the cultures were initiated by single germinated spores or by mass transferred inoculum. Those colonies which did sporulate on MA or MEA formed abundant sporodochia on CLA but not perithecia. CLA was also used to identify F. graminearum (G. zeae) from maize seeds or seedlings by direct plating of these structures after surface disinfection. Full descriptions of the Fusarium colonies on the various media used are presented. Fusarium survival in seed during storage depended upon seed moisture content (SMC) and storage temperature. F. graminearum was eliminated from seed at 14% SMC stored at 30°C and 25°C after 3 or 6 months storage, respectively, but survived at low levels (1-5%), together with F. subglutinans (1-7%), F. poae (1-2%) at these temperatures and 10% SMC. F. subglutinans and F. poae in seeds at 14% SMC did not survive after 9 months storage at 30°C. In seed stored at 5°C, Fusarium spp. infection levels did not decline after 12 months of storage at both 10 and 14% SMC. These results suggest a possible control strategy for producing Fusarium free seed, providing seed moisture content is not greater than 10%. At a storage temperature of 30°C, the post-storage germination of seed at 14% SMC had dropped to under 10% within 3 months, but seed at 10% SMC maintained its germination (88-97%) throughout the storage trial. After 12 months seed storage at 5°C (sealed storage) or 25°C (open storage), mycotoxin levels were similar to pre-storage levels. The requirements of Koch's postulates were fulfilled in demonstrating that seed-borne F. graminearum was transmitted from maize seeds to seedlings under aseptic conditions in a glasshouse maintained at a temperature of 14°C to 17°C. The mean transmission rate (48%) was similar to the original seed-borne inoculum which suggests that under favourable environmental conditions, the pathogen will be effectively transferred from the seed to seedlings. F. graminearum had little effect on seedling emergence or survival, but was associated with a high percentage of seedlings with scutellum-mesocotyl/scutellum-main root lesioning. In the field, F. graminearum was consistently isolated from seedlings, but seed transmission could not be confirmed because of the presence of soil-borne inoculum, ie the pathogen was isolated from up to 37% of seedlings from a seed lot which carried only 1% seed-borne inoculum. F. subglutinans was also proved to be seed transmitted under the same glasshouse conditions as described for F. graminearum. The significance of surface-borne inoculum of this pathogen was demonstrated in that the mean transmission rate for non-surface disinfected seed lots was 81%, whereas it was only 7% for surface disinfected seed lots. F. subglutinans was associated mainly with 'above sand level' seedling infection (coleoptile-node infection, leaf/shoot blight, shoot wilt and seedling stunting). However, F. subglutinans was rarely detected in seedlings from the field, possibly because of the antagonistic effects of mycopathogenic fungi such as Gleocladium roseum. These results are discussed, particularly in relation to the significance of F. graminearum and F. subglutinans as seed-borne pathogens of maize, and the difficulties inherent in the identification of Fusarium spp. following seed health testing. It is likely that these seed-borne Fusarium spp. are more important because of their association with mycotoxins, than with any effects they have as an inoculum source for diseases of maize.
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    Physical changes in maize (Zea mays L.) grains during postharvest drying : a thesis presented in partial fulfillment of the requirements for degree of Doctor of Philosophy in Seed Technology at Massey University
    (Massey University, 2000) Kim, Tae Hoon
    Stress cracking due to high temperature drying has been of concern to the maize industry because it can lead to increases in broken grain and fine material during subsequent handling. In this study, several factors affecting physical characteristics of maize grain, particularly those related to stress cracking, were investigated. In the first year (1995-1996), the effects of several preharvest factors; hybrid, nitrogen, harvest grain moisture content, and postharvest drying factors including drying temperature and relative cooling rate on physical attributes and stress cracking in grain were investigated. Grain hardness (hard to soft endosperm ratio (H/S ratio)) was significantly affected by the interaction between hybrid and nitrogen. The effect of drying temperature and harvest moisture on drying time was dominant, while drying rate was significantly affected by hybrid and drying temperature. The effect of cooling rate on stress cracking and stress crack index (SCI) stood out among the main effects. At the lowest cooling rate of 0.23 (°C/°C/min.)•10-2, checked stress cracking (checking) was minimal, and SCI was less than 100. However, at higher cooling rates from 0.55 to 1.11 (°C/°C/min.)•10-2, grains had more than 25% multiple stress cracking, regardless of the levels of hybrid, nitrogen, harvest moisture and drying temperature. The predicted SCI for the three hybrids reached a maximum around at 0.75 (°C/°C/min.)•10-2, cooling rate, irrespective of levels of nitrogen and drying temperature. In the second experiment (1996-1997), the effects of grain hardness and morphological factors (grain size and shape) at a single grain drying rate and the development stress cracking over time were investigated. The re-parameterized Morgan-Mercer-Flodin (MMF) model successfully predicted the increasing rate (κ) and the maximum value (α) of percentage checking in various sizes, shapes and hardness of grains time after drying. From the data analysis, the maximum value of checking (α) showed a significant correlation with grain length (r = -0.707), thickness (r = 0.620), roundness (r = 0.703) and the shortest diffusion pathway (SDP; r = 0.627). While, the increasing rate (κ) of percentage checking with time after drying was significantly correlated with grain bulk density (r = -0.564), hardness ratio (r = -0.611) and drying rate (r = 0.551), and to a lesser extent (r > 0.35), with the grain size parameters including hundred-grain weight, grain length, and width. Based on this result, it was suggested that removing small and rounded grains could reduce checked stress cracking by up to 40 to 50% in some dent maize hybrids. In addition, the standardized multiple regression for single grain drying rate according to H/S ratio and grain weight accounted for from 65 to 74% of the variation. Tempering grain at high temperatures reduced stress-cracked grains significantly. However, the effect of tempering on stress cracking in the hard grain hybrid was small. In the 1997-1998 experiment, a breakage tester (HT-I drop tester) was developed and single grain breakage at various grain temperatures and times after drying was determined. Both hard and soft maize hybrids had minimal breakage at high grain temperatures (78 to 110°C), while decreasing grain temperature increased breakage exponentially. This indicated that grain temperature should be considered as a co-factor for measuring grain breakage. After drying at both 60°C and 120°C, the percentage breakage measured at ambient temperature increased rapidly during cooling in air at an ambient temperature of 20°C and a relative humidity around 65-70%. Breakage reached a maximum after about 10 minutes from the start of cooling. A Mitscherlich function was used to describe the chronological development of percent grain breakage and the analysis of the function parameters for the extent (maximum) and rate of breakage indicated that there was a significant interaction between hybrid and drying temperature for the development of grain breakage after drying. In conclusion, the MMF and Mitscherlich models described stress cracking and grain breakage during drying and cooling of maize grain. These studies provide valuable information to grain industries to assist with minimizing grain damage during drying.