This thesis describes aspects of the biology of Mycocentrospora acerina (Hart.) Deighton and their implications for caraway ( Carum carvi L.) cultivation. The biology of M. acerina under field conditions was not yet described satisfactorily. More is known of M. acerina as the causal agent of liquorice rot, a post-harvest disease in carrots. Various studies in caraway were performed in the period from 1990 until 1995 with the aim to obtain information on M. acerina and to generate a set of crop protection methods based on this information. Chapter 1 provides an introductory overview.
Chapter 2 reports on the dispersal of conidia of M. acerina in caraway field trials. A Burkard spore trap, rotorods, inverted Petri dishes containing sucrose agar and rain gauges were used to trap conidia of M.acerina . Sporulation was stimulated by rainfall (≥2 mm) and moderate temperatures (around 15°C). Solar radiation had a negative effect on sporulation. Hardly any conidia were found in the spore traps on rainless days. Short distance (≤9 m) spread of M.acerina was mainly caused by splash dispersal of its conidia. Trap plants at 0 to 4 m from the inoculum source were readily infected under moist conditions. Beyond 9 m from an inoculum source no infection of caraway trap plants was found. Trap plants at 9 m from an inoculum source were infected in one out of three seasons only. Long distance (> 9 m) spread could not be demonstrated by the techniques used in this study. The results suggest that, usually, a caraway field is infected by inoculum sources within that field.
Chapter 3 deals with the effect of mechanical injury on disease incidence, incubation period and lesion development rate on caraway roots following infection by M. acerina . After inoculation with M. acerina , disease incidence of injured roots was significantly higher than of non-injured roots under laboratory conditions. The incubation period of M. acerina was significantly shorter on injured roots than on non-injured roots. The incubation period shortened with increasing root injury level. Younger injured roots tended to be more resistant to M. acerina infection than older injured roots, as expressed by longer incubation periods. The lesion development rate was, on average, higher on heavily injured roots than on non-injured or slightly injured roots. The lesion development rate remained fairly constant after the first appearance of the symptoms on the caraway root, until the whole root was colonized. Caraway roots carefully dug up in autumn frequently showed injuries enabling M. acerina to penetrate the roots. However, the correlation between root injury and root rot after cold storage was weak. Injury of roots had a stimulating effect on infection by and development of M. acerina , but roots without wounds could be infected too.
Seed transmission of M. acerina in caraway is demonstrated in Chapter 4. Seed transmission efficiency varied between 27 and 62%. Methods to quantify M. acerina seed infection in caraway are presented and discussed. Seed infection levels were usually less than 3%, but infection levels up to 50% were found. M. acerina remained viable in caraway seeds for at least three years. Results suggest that infected caraway seed can be an inoculum source for the development of anthracnose in caraway crops. The importance of seed infection for long distance dissemination and for the onset of an epidemic is discussed.
The effect of seed infection of caraway by M. acerina on crop establishment and yield was studied in field experiments described in Chapter 5. High seed infection levels hampered crop establishment of caraway and limited the number of plants producing a root diameter large enough to permit flowering in the next year.
Chapter 6 reports on the search for inoculum sources of M. acerina on caraway. Obvious suspects are cover crops of biennial caraway and preceding crops of annual caraway. Other suspects are weeds in or alongside the field. Finally, survival structures of the fungus, chlamydospore chains, packed in plant debris or naked, are suspected. M. acerina is able to infect many plant species, including cover crops of caraway such as spinach for seed production and peas. However, the agronomical suitability of a crop to serve as a cover crop of biennial caraway proved to be a more important factor in determining caraway yield than the susceptibility of the cover crop to M. acerina . This finding was corroborated by the fact that spinach and peas as preceding crops had no significant effects on M. acerina development in spring caraway sown the next year. Dill, barley and four weed species were found as new hosts of M. acerina . The role of weed hosts, susceptible crops and plant debris in the survival of the fungus in years without caraway is discussed. Caraway sown on soil containing infested caraway straw, infested debris of other plant species or chlamydospores grown in pure culture, became infected by M. acerina . Only high inoculum densities of chlamydospores in the soil caused severe damping-off of caraway seedlings. The opportunity for disease management by agronomical means is quite limited.
Chapter 7 describes the effect of some cultivation measures on anthracnose development in caraway. A reduction of leaf wetness duration was positively correlated with a decrease of disease severity. Lodging and higher plant density prolonged leaf wetness duration. Disease incidence and severity of anthracnose were reduced by crop management activities minimizing leaf wetness duration. Reduction of nitrogen levels reduced the risk of anthracnose development in spring and biennial caraway. Decreasing the sowing rate from 8 to 4 kg ha -1resulted in a lower disease severity and an increase of seed yield in spring caraway, but not in biennial caraway. In biennial caraway disease severity decreased with wider row spacing. A damage threshold between 6% and 12% disease severity is proposed. Positive financial results of crop management activities are indicated.
Relationships between scientific aspects described in the previous chapters and the applied research described elsewhere are discussed in Chapter 8. Problems with anthracnose of caraway occur after long periods of rainfall. The disease cannot be prevented by any single crop protection measure. However, some measures reduce the occurrence of M. acerina. Reducing the risk of yield loss due to this pathogen should begin with 1) a disease-free start to delay the build-up of inoculum (disease-free seeds, choice of an uncontaminated field), 2) slowing down the epidemic in spring by taking measures to avoid lodging and to limit leaf wetness duration in a crop (reduction of sowing rate, intermediate row spacing and reduction of nitrogen fertilization), and 3) seed dressing can retard the early infection of seedlings, but it is illegal in the Netherlands. Fungicides against white mould ( Sclerotinia sclerotiorum ) sometimes reduce anthracnose in caraway, but results cannot be guaranteed.
|Qualification||Doctor of Philosophy|
|Award date||10 Jun 1998|
|Place of Publication||S.l.|
|Publication status||Published - 1998|
- plant pathogenic fungi
- carum carvi
- plant protection
- pest control
- disease control
- mycocentrospora acerina