Parry's disease of pears is similar to pear decline and is associated with mycoplasma-like organisms transmitted by Cacopsylla pyricola

Mycoplasma-like organisms (MLOs) were detected by fluorescence microscopy in a number of pear cultivars and pear seedlings growing under a range of different conditions. In the cultivar Conference, MLOs were consistently associated with symptoms of Parry's disease, a decline-like disease of young trees with quince rootstocks. MLO-free pear seedlings rapidly became infected when they were planted outside. Experimental transmission of MLOs to pear seedlings and Conference trees was achieved using pear psyllids, caught outside or raised on infected plants under controlled conditions. Conference trees in an orchard trial remained free from the severe spring symptoms of Parry's disease when they were protected from feeding insects during spring of the previous year. MLOs were graft transmissible, but were not perpetuated by the standard propagation practices of budding or grafting when quince rootstocks were used. Parry's disease appears to be similar to pear decline, an MLO-induced disease well established in several other parts of the world. It is suggested that Parry's disease should be referred to as pear decline.     

Variation in the resistance of Vicia faba to Ascochyta fabae in relation to disease development in field trials

The epidemic development of Ascochyta fabae in artificially infected field trials was studied in order to define resistance criteria and assess the variation in resistance of infected lines of faba bean. Large differences were demonstrated among French winter lines during the 1985/86 and 1986/87 growing seasons. All the scoring systems allowed the differentiation of very susceptible and resistant material, but disease indices, which account for the number and intensity of lesions, were more precise. The different phases of the epidemic are described on the basis of the variation in disease incidence on foliage and on pods during the 1986/87 cropping season in Brittany. There were three phases in disease development: initiation, spread throughout foliage, and pod infection, and these were all influenced by host resistance. On a very susceptible line, the disease spread was seen as an increase in the number and size of lesions and rapidly developed up the plant. Low disease indices observed on the resistant line 29 H were due to the small size and low frequency of lesions, resulting in a reduced area of foliage affected.     

Cropping systems for the management of phytonematodes

Damage caused by nematodes is one of the limiting factors in crop production. Traditional nematode management is based on the use of crop rotations, resistant cultivars, nematicides, or combinations of these methods. For a crop like peanut (Arachis hypogaea), cultivars resistant to root-knot nematodes are not available. There are soybean (Glycine max) cultivars resistant to some of the species of root-knot nematodes (Meloidogyne spp.); however, most fields have nematode infestations composed of mixtures of species. Research at Auburn has shown that tropical crops can be used effectively in rotation to manage nematode problems. Rotations with American jointvetch (Aeschynomene americana), castor (Ricinus communis), hairy indigo (Indigofera hirsuta), partridge pea (Cassia fasciculata), sesame (Sesamum indicum), and velvetbean (Mucuna deeringiana) have resulted in good nematode control and increased yields of peanut and soybean. Some crops (castor, sesame) are considered ‘active’ in that they produce compounds that are nematicidal, whereas others (e.g. corn, sorghum) are simply non-host, that is, ‘passive’.     

Genetic engineering of insect viruses for insect biological control

Insect viruses can be exploited for the biological control of insect pests and serve as well as vehicles for the introduction of ‘foreign genes’ into insect cells and insects. The present work describes some of the molecular events that take place during the replication of theGalleria mellonella densonucleosis virus in the insect host. Viral RNA and protein synthesis were monitored. An understanding of the virus biology is essential for further implementation of these viruses for biological control purposes.     

Indol-3-ylacetic acid- and calmodulin-regulated Ca2+-ATPase: A target for the phytotoxic action of hexachlorocyclohexane

In rice seedlings, the organochlorine insecticide hexachlorocyclohexane (HCH) and its four major isomers—α,β, γ, and δ—have been shown to elicit their phytotoxic action by interacting with indol-3-ylacetic acid (IAA)-regulated growth and Ca²⁺-ATPase activity. When rice seedlings were grown in the presence of 0.34 mM HCH, seedling vigour was reduced to 33% of the control. A similar effect was observed when seedlings were grown in the presence of the γ and δ isomers, but not with the α and δ isomers of HCH. This reduced vigour could be restored by treating the seedlings with 100 nM IAA, suggesting that HCH and its isomers (γ and δ) limit either IAA synthesis or action or both. In a microsomal fraction from rice seedlings grown in the presence of either HCH or its isomers, the calmodulin-regulated Ca²⁺-ATPase activity was inhibited as follows: technical HCH = 46%, α isomer = 80%, β isomer = 72%, γ isomer = 65% and δ isomer = 62% of the control value, respectively. TLC analysis suggested that the various isomers of HCH are metabolised by the plant, except for the δ isotner, which accumulated. This isomer, along with the degraded products of other isomers, may be responsible for the phytotoxic action of HCH.     

Development of glasshouse pests of cucumber under different conditions,in relation to possibilities for their biological control1

Experiments were performed under controlled conditions on the development of four cucumber pests (Trialeurodes uaporariorum, Tetranychus urticae, Aphis gossypii and Thrips tabaci). The main variables were temperature, photoperiod and light intensity. The effect of these factors on the natural enemies Encarsia formosa and Amblyseius mckenziei was also assessed. Comparison of the results allows recommendations on manipulation of the glasshouse environment to favour biological control.     

Variation for pathogenicity among isolates of bean common mosaic virus in Africa and a reinterpretation of the genetic relationship between cultivars of Phaseolus vulgaris and pathotypes of BCMV

Bean common mosaic virus (BCMV) isolates were collected from crops of Phaseolus vulgaris (bean) and from wild legume species in 13 African countries. Isolates of pathotype VIa from both beans and wild legume species were predominant in central, eastern and southern Africa. Isolates of pathotypes I, III, IVa, IVb and Va were also found. Some isolates did not conform to previously published pathotypes, and therefore represent records of novel pathotypes. The susceptibility of various wild legume species to BCMV was investigated and isolates of the virus obtained from Crotalaria incana, Rhynchosia sp., Macroptilium atropurpureum and Cassia occidentalis (synonym Senna occidentalis) were aphid-transmitted both from P. vulgaris to their original host species and to P. vulgaris. Isolates of BCMV from wild legume species were seed-transmitted in bean and in several other legume species. The natural occurrence of BCMV in wild legume species in Africa is probably a significant factor in the ecology and epidemiology of the virus and possibly the evolution of isolates of the 'A' serotype which induce necrotic reactions in cultivars carrying the I gene for resistance. The occurrence of viruses other than BCMV from P. vulgaris and other legume hosts is also reported. The gene-for-gene model described by Drijfhout (1978) is reinterpreted to explain the variation for pathogenicity, and it is proposed that there may be genes which control the temperature sensitivity of necrosis in combination with the I gene.     

Losses in grain yield of winter crops from Lolium rigidum competition depend on crop species, cultivar and season

The competitive abilities of eight winter crops were compared against Lolium rigidum Gaud, (annual ryegrass), an important weed of southern Australia, as a potential strategy to suppress weeds and reduce dependence on herbicides. Two cultivars of each species were chosen to represent the range of competitive ability within each crop and grown in field experiments in 1992 and 1993. The order of decreasing competitive ability (with the ranges of percentage yield reduction from L. rigidum at 300 plants m^?2 in parenthesis) was as follows: oats (Avena sativa L.), 2–14%; cereal rye (Secale cereale L.), 14–20%; and triticale (×Triticosecale), 5–24%; followed by oilseed rape, (Brassica napus L.), 9–30%; spring wheat (Triticum aestivum L.), 22–40%; spring barley (Hordeum vulgare L.), 10–55%; and, lastly, field pea (Pisum sativum L.), 100%, and lupin (Lupinus angustifolius L.), 100%. Differences in competitive ability of cultivars within each species were identified, but competition was strongly influenced by seasonal conditions. Competition for nutrients (N, P and K) and light was demonstrated. L. rigidum dry matter and seed production were negatively correlated with grain yield of the weedy crops. More competitive crops offer the potential to suppress grass weeds while maintaining acceptable grain yields. Ways of improving the competitive abilities of grain legume crops are discussed.     

The use of polymerase chain reaction to detect latent infection of Clavibacter michiganensis subsp. michiganensis in tomato seedlings

A method was developed for routine analysis to detect latent infection of Clavibacter michiganensis subsp. michiganensis in samples from tomato seedling lots, before transplant, using polymerase chain reaction. In samples of 300 stem segments 1 cm long, the sensitivity threshold of the method was estimated at around 1.1 × 10³ corynebacteria or 0.33% of latently infected stems. This method was shown to have a good specificity.