The significance of ecology in the development of Verticillium chlamydosporium as a biological control agent against root-knot nematodes (Meloidogyne spp.)

A thorough understanding of the interactions which occur between nematode parasites and nematode pests and the influence of biotic and abiotic factors on these interactions, is essential in the development of biological control agents for nematodes. The aim of this study was to develop a particular isolate of the nematophagous fungus Verticillium chlamydosporium as a biological control agent for root-knot nematodes. The work has gained insight into some of the key factors which govern the efficacy of the fungus as a biological control agent. The development of a semi-selective medium for V. chlamydosporium made it possible to study the growth, survival and spread of this fungus in non-sterile soil and on different parts of the root system as affected by soil-type, fungal density, nematode density, nematode species, temperature and watering.The V. chlamydosporium isolateThe isolate of V. chlamydosporium used in these studies was effective against Meloidogyne incognita, M. javanica , M. arenaria and M. hapla , and had no plant pathogenic characteristics. The fungus can be regarded as a typical egg-parasite, a characteristic which made it unsuitable for preventing initial nematode damage. However, effective population control in excess of 80% gave significant damage control after more than one nematode generation. After one generation of nematodes, population control achieved with the fungus was comparable with a nematicide treatment of aldicarb equivalent to 3.75 kg a.i./ha.The importance of ecological factors on the efficacy of V. chlamydosporiumThe efficacy of V. chlamydosporium as a biological control agent against rootknot nematodes is governed by four key factors:(a) Fungal establishment on the rhizoplane.(b) The proportion of egg-masses of Meloidogyne spp. exposed on the rhizoplane.(c) Rate of fungal growth relative to nematode development.(d) Spread of the fungus through soil.Fungal establishment on the rhizoplane : This is dependent on:1. Soil type . Mineral soils support less fungal proliferation than organic ones. This results in a slower build up of fungus on the rhizoplane when plants are grown in mineral soil compared with plants grown in organic soil.2. Application rate . This determines the initial fungal establishment on the rhizoplane. Presumably, because there are more nutrients available for the growth of a small initial fungal inoculum on the rhizoplane than for a large one, initial differences in fungal establishment on the roots tend to disappear after several weeks.3. Nematodedensity/gailing . Root galls induced by Meloidogyne spp. stimulate fungal growth on the gall surface.4. The host plant . Verticillium chlamydosporium differs in its ability to colonise the rhizoplane of different host plants, with for example roots from tomato, maize, cabbage and potato plants being readily colonised by the fungus, while roots from pepper, sorghum, soy-bean, pigeon pea and cotton plants are far less readily colonised.Exposure of egg-masses of Meloidogyne on the rhizoplane . This depends on:1 . Nematode density . Galling on tomato plants is much more severe when there is a high density of nematodes feeding in the roots, resulting in a proportion of of egg-masses staying embedded in the gall tissue at high nematode densities. These embedded egg-masses are physically protected from fungal attack.2. Temperature . Low temperatures (around 20°C) result in the induction of larger galls compared with those produced at higher temperatures (25°C-30°C). At 20°C, the egg-masses produced by M. incognita and M. javanica are in generalsmaller than those produced between 25°C and 30°C. The combination of these effects results in fewer egg-masses being exposed at lower temperatures.3. Nematode species . Galls induced by M. arenaria are larger than those induced by M. incognita or M. javanica . Depending on temperature this means that more egg-masses might stay embedded in gall-tissue when roots are infested by M. arenaria than with infestations of M. incognita or M. javanica .4. Host plant . There are marked differences in gall-size between different host plants infested with Meloidogyne. This undoubtedly has consequences for the proportion of egg-masses being exposed on the root-surface. However, data on this subject are not presented in this thesis.Rate of fungal growth relative to nematode development . This depends on:1 . Temperature . At temperatures below 25°C V. chlamydosporium is able to infect eggs before they mature and contain second-stage juveniles. At temperatures above 25°C nematode eggs develop faster than the fungus can infect them. This results in a proportion of eggs developing into fully embryonated eggs and juveniles at those temperatures. Juveniles and fully embryonated eggs are far less susceptible to fungal infections and it can be assumed that they have escaped further fungal attack. At 30°C. this may result in more than 40% of eggs escaping fungal infection.2. Aeration . Aeration influences the rate with which nematodes develop as well as the extend of fungal growth. It seems that, when oxygen is in short supply, the fungal growth rate is slower than nematode development. This factor however needs further investigation.Spread through soil . Numbers of colony forming units (cfu) can increase rapidly in soil when V. chlamydosporium is introduced as a chlamydospore inoculum into non sterile soil. Increase in numbers of cfu in soil is related to the soil temperature, and is most likely explained by the formation of conidia. These conidia can be moved by water percolating through soil. Roots of tomato plants became extensively colonised in this way up to 20 cm away from the original inoculum source, nine weeks after inoculation. This resulted in 100% of exposed egg-masses being colonised by the fungus. However, spread of conidia is dependent on:1 Watering . When the top 10 cm of the soil was inoculated with fungus, conidia spread more rapidly and more evenly when water was added from the top in comparison with watering from below.2. Water filled pores . When surplus water was allowed to drain out of the water saturated soil, subsequent waterings had no significant effect on further movement of conidia.3. Soil type . This factor needs further investigation, but it is likely that movement of conidia is greater in soils with a coarse soil texture and large pores than in soils with a fine soil texture and narrow pores.The practical implications of the studyThe ecological factors which govern the efficacy of V. chlamydosporium as a biological control agent against Meloidogyne were studied in the glasshouse. It was encouraging that introduction of V. chlamydosporium into field soil in a micro-plot test resulted in more than 90% population reductions of M. hapla on tomatoes, showing that data obtained from pot-tests were relevant in the field. The nematicide aldicarb (application rate: 3.75 kg a.i./ha) applied in combination with the fungus had no detrimental effects on fungal establishment in soil or on the root-surface. Fungal efficacy was therefore not influenced by the nematicide, resulting in greater control (98%) when both control measures were combined.The ecological studies presented in this thesis have practical implications for the use of V. chlamydosporium in the control of root-knot nematodes. The insights into the ecology of the soil might also be useful in the development of cultural practices to enhance soil suppressiveness in field soils

Insights from research on children with a familiar risk for dyslexia

Al langer is bekend dat dyslexie in families voorkomt. Kinderen met een ouder met dyslexie hebben een grotere kans om ook lees- en spellingproblemen te ontwikkelen. In familiaire risico (FR) studies naar dyslexie worden kinderen met familiair risico op dyslexie voor langere tijd gevolgd. Dergelijke studies geven interessante inzichten over de oorzaken en ontwikkeling van dyslexie. In Nederland is recent een grootschalige FR-studie afgerond, het Dutch Dyslexia Program. In deze bijdrage gaan we in op de bevindingen die dit onderzoek heeft opgeleverd op een tweetal terreinen. In de eerste plaats zijn er bevindingen over de verschillen tussen FR- kinderen met en zonder dyslexie, en controle kinderen zonder familiair risico. Zoals verwacht, hadden de FR-kinderen met dyslexie een achterstand op een scala aan leesgerelateerde cognitieve factoren. Interessant was dat ze ook een wat hoger familiair risico hadden dan de FR-kinderen zonder dyslexie: hun ouders lazen nog wat slechter. De FR-kinderen zonder dyslexie hadden milde problemen met lezen en spellen en een kleine achterstand op de voorlopers van het lezen. Het tweede terrein betreft bevindingen over de invloed van FR op de ontwikkeling van geletterdheid, technisch en begrijpend lezen. Kinderen werden gevolgd vanaf 4-jarige leeftijd tot ongeveer 12 jaar, halverwege groep 8. In longitudinaal perspectief had een familiair risico een negatieve invloed op de ontwikkeling van technisch en begrijpend lezen. De invloed op technisch lezen liep gedeeltelijk via de voorlopers van het lezen, maar daar kwam nog een direct effect bij. Onverwacht, bleek familiair risico ook een direct effect op begrijpend lezen in groep 8 te hebben. Familiair risico had geen effect op de ontwikkeling van de woordenschat, naast technisch lezen de andere pijler van begrijpend lezen. De theoretische en praktische betekenis van deze bevindingen worden besproken

Multiyear trend in reproduction underpins interannual variation in gametogenic development of an Antarctic urchin.

Ecosystems and their biota operate on cyclic rhythms, often entrained by predictable, small-scale changes in their natural environment. Recording and understanding these rhythms can detangle the effect of human induced shifts in the climate state from natural fluctuations. In this study, we assess long-term patterns of reproductive investment in the Antarctic sea urchin, Sterechinus neumayeri, in relation to changes in the environment to identify drivers of reproductive processes. Polar marine biota are sensitive to small changes in their environment and so serve as a barometer whose responses likely mirror effects that will be seen on a wider global scale in future climate change scenarios. Our results indicate that seasonal reproductive periodicity in the urchin is underpinned by a multiyear trend in reproductive investment beyond and in addition to, the previously reported 18–24 month gametogenic cycle. Our model provides evidence that annual reproductive investment could be regulated by an endogenous rhythm since environmental factors only accounted for a small proportion of the residual variation in gonad index. This research highlights a need for multiyear datasets and the combination of biological time series data with large-scale climate metrics that encapsulate multi-factorial climate state shifts, rather than using single explanatory variables to inform changes in biological processes