Characterization of host tolerance to Striga hermonthica

One of the most promising control options against the parasitic weed Striga hermonthica is the use of crop varieties that combine resistance with high levels of tolerance. The aim of this study was to clarify the relation between Striga infestation level, Striga infection level and relative yield loss of sorghum and to use this insight for exploring the options for a proper screening procedure for tolerance. In three pot experiments, conducted in Mali (2003) and The Netherlands (2003, 2004), four sorghum genotypes were exposed to a range of Striga infestation levels, ranging from 0.0625 to 16 seeds cm−3. Observations included regular Striga emergence counts and sorghum grain yield at maturity. There were significant genotype, infestation and genotype × infestation effects on sorghum yield. The relation between infestation level and infection level was density dependent. Furthermore, the relation between Striga infection level and relative yield loss was non-linear, though for the most resistant genotype Framida only the linear part of the relation was obtained, as even at high infestation levels only moderate infection levels were achieved. The results suggest that for resistant genotypes, tolerance can best be quantified as a reduced relative yield loss per aboveground Striga plant, whereas for less resistant genotypes the maximum relative yield loss can best be used. Whether both expressions of tolerance are interrelated could not be resolved. Complications of screening for tolerance under field conditions are discussed

Effects of N, P and K on Striga asiatica (L.) Kuntze seed germination and infestation of sorghum

Sorghum (Sorghum bicolor (L.) Moench) plants were grown in pots with 12.5 and 50 mg applied N kg−1 soil. With an increase of soil N, the Striga asiatica (L.) Kuntze infestation, as well as the sorghum shoot dry matter losses due to infestation, decreased. The relative differences in stimulant capacity to induce Striga seed germination among the four sorghum genotypes were not consistent over the 0 to 150 mg N 1−1 range. The sorghum root exudate was considerably more active at 0 mg N 1−1, than at 30 mg N 1−1, and the stimulant produced at 150 mg N 1−1 failed to induce Striga seed germination. Presence of N in the growth medium considerably reduced the effectiveness of the stimulating substance produced by sorghum roots, whereas K promoted stimulant activity only in the absence of N. The presence or absence of P in the growth medium did not affect Striga seed germinability, probably due to the inability of this element to interfere with the production or activity of the stimulating substance from the host plants. It can be concluded, therefore, that sorghum plants seem to produce active root exudate only in conditions of N deficienc

Interactions between butterfly pea (Clitoria ternatea) and rubber vine (Cryptostegia grandiflora)

The arutors studied the impact of a forage legume, butterfly pea, on rubber vine at the early establishment phase under seven planting combinations at three nitrogen fertiliser levels. In pure stands, both species increased their shoot and root dry weight yield in response to nitrogen but rubber vine exhibited the greater response. In mixed stands, rubber vine and butterfly pea did not compete with each other at any nitrogen level. An over-yielding response resulted in all mixture combinations in terms of shoot and root yields. Total shoot and root mass of mixed stands significantly out-yielded their highest yielding pure stands by 8% and 27% respectively, suggesting that butterfly pea not only failed to reduce shoot and root growth of rubber vine, but actually improved its growth performance. Consequently, the introduction of butterfly pea to suppress rubber vine is not warranted

Priority lists for weed research in the wet- and dry- tropics of north Queensland

A participatory decision making approach was used to prioritize weed research for the wet- and dry-tropics of north Queensland using stakeholders, an expert panel, external reviewers, and the computerized priority-setting system (QDNRM MODSS). From 246 weeds identified, 53 weeds were found to be of major concern. A score was developed for each weed, which accounted for its priority in individual shires and the number of times it was prioritized across the 47 shires. The 22 dry- and 31 wet-tropics weeds were scored by an expert panel against 12 criteria based on economic, environmental and social impacts and current research knowledge with higher scores for greater impact and greater amount of needed research. Several ‘what if’ scenarios were run through QDNRM MODSS by altering the relative importance of criteria. The best criterion for prioritizing weed research was when impact was given higher importance than research needs. Mikania micrantha, Chromolaena odorata, Mimosa invisa, and Eupatorium catarium were the top four weeds of the wet-tropics, and Prosopis spp., Parthenium hysterophorus, Jatropha gossypiifolia, and Cryptostegia grandiflora for the dry-tropics. Priority lists will be used to decide the order in which weeds should be researched and what type of research needed to be done

Damage potential of an introduced biological control agent Agonosoma trilineatum (F.) on bellyache bush (Jatropha gossypiifolia L.)

The seed-feeding jewel bug, Agonosoma trilineatum (F.), is an introduced biological control agent for bellyache bush, Jatropha gossypiifolia L. To quantify the damage potential of this agent, shadehouse experiments were conducted with individual bellyache bush plants exposed to a range of jewel bug densities (0, 6 or 24 jewel bugs/plant). The level of abortion of both immature and mature seed capsules and impacts on seed weight and seed viability were recorded in an initial short-term study. The ability of the jewel bug to survive and cause sustained damage was then investigated by measuring seed production, the survival of adults and nymph density across three 6-month cycles. The level of seed capsule abortion caused by the jewel bug was significantly affected by the maturity status of capsules and the density of insects present. Immature capsules were most susceptible and capsule abortion increased with jewel bug density. Similarly, on average, the insects reduced the viability of bellyache bush seeds by 79% and 89% at low and high densities, respectively. However, sustaining jewel bug populations for prolonged periods proved difficult. Adult survival at the end of three 6-month cycles averaged 11% and associated reductions in viable seed production ranged between 55% and 77%. These results suggest that the jewel bug has the potential to reduce the number of viable seeds entering the soil seed bank provided populations can be established and maintained at sufficiently high densities