Evidence of sexual reproduction events in the dagger nematode Xiphinema index in grapevine resistance experiments under controlled conditions

The dagger nematode Xiphinema index has a major economic impact due to its transmission of Grapevine fanleaf virus (GFLV) to grapevines. This vector nematode, which was introduced into Western countries from the Middle East together with the domesticated grapevine, mostly reproduces by meiotic parthenogenesis, but microsatellite multilocus genotype (MLG) analysis has revealed the occurrence of rare sexual reproduction events in field conditions. In a previous six-year study under controlled conditions, we evaluated the durability of resistance (R) to X. index in accessions derived from a muscadine R source and reference accessions. In this previous study, we used an equal-proportion mixture of four lines (from Spain, Italy, Greece and Iran) representative of X. index diversity as the inoculum, and we collected random samples in three-, four-, five- and six-year-old vines. Here, we genotyped the individuals from these samples using MLG technique, and we analyzed the changes in line frequency and the occurrence of sexual reproduction events between lines over time. The nematode lines differed in aggressiveness and hybrids between lines were detected at a low, but apparently increasing rate. Hybridization events were recovered in all accessions, regardless of resistance status and propagation type. Finally, our data provide the first evidence of sexual reproduction in the nematode X. index under controlled conditions.

Modern Breeding Approaches for Durable Resistance Against the Parasitic Plant Striga

Crop losses caused by parasitic plants of the genus Striga pose a great danger to the livelihoods of millions of smallholder farmers in Africa. The parasite attaches to host crops and siphons nutrients leading to severe retardation and crop death. Controlling Striga is difficult because of the parasite’s ability to produce large amounts of seeds that can remain dormant in the soil for decades – only germinating in response to chemical cues (strigolactones) from the host. In recent years, breeding crops for host-based resistance has been prioritized. However, such programs have not taken into account Striga’s ability to overcome host resistance. As a result, introduced resistance fails because of increased Striga virulence (infection severity). This article reviews technologies for a new paradigm in Striga resistance breeding that incorporates host resistance breeding with well-informed knowledge of parasite resistance in order to ensure durability of resistance.

Preliminary Results on Behavior of Transgenic and Conventional Plum to Natural PPV Infection in a New Field Trial

To assess the durability of resistance to PPV of HoneySweet transgenic plum harbouring plum pox virus (PPV) capsid gene, a new field trial was recently established in Romania at Fruit Research & Development Station Bistrita. Stanley and Reine Claude d’Althan cvs. are used as control. PPV inoculum source is secured within the experimental plot. PPV monitoring was made by visual observation of symptoms development on leaves and by serological (DAS-ELISA) and molecular (IC/-RT-PCR) testings. Three years after planting PPV was observed and detected only on Stanley. 'HoneySweet' and Reine Claude d’Althan cultivars remained PPV free.

Historical Durability of Resistance to Wheat Diseases in Kansas

In Kansas, estimated annual losses from wheat diseases averaged 16% from 1976 through 1988; however, losses have declined to 10% in recent years. This decline is mainly due to emphasis on developing cultivars with resistance to important diseases. Data from annual KSU extension publications were used to track disease ratings over time for individual wheat cultivars to eight diseases to draw inferences on the durability of deployed resistance. The duration of durability of resistance to leaf rust has been short for some cultivars but moderate for others. Conversely, the duration of durability of resistance to stem and stripe rusts has been long, although a recent virulence shift in the stripe rust pathogen indicates only moderate duration. Duration of durability of resistance toward tan spot, Septoria tritici blotch, wheat soilborne mosaic, and wheat spindle streak mosaic has been long. Resistance to wheat streak mosaic has been of long duration in some cultivars but others have shown only moderate duration of durability. The effort by breeders to produce cultivars with resistance to diseases in Kansas has resulted in an annual savings of about $58 million. Just as important as the incorporation of resistance into those cultivars is the moderate to long duration of durability of the resistance contributing to effective management of these diseases. Accepted for publication 22 June 2011. Published 2 August 2011.

Apple powdery mildew caused by Podosphaera leucotricha: some important features of biology and epidemiology

In this review, some important features of biology and epidemiology are summarised for apple powdery mildew (Podosphaera leucotricha). In the first part of the review, the geographical distribution or the pathogen are discussed, then the morphology and taxonomy of the causal organism are described. Disease symptoms or apple powdery mildew are also shown and then host susceptibility/resistance is discussed in relation to durability of resistance. In the second part of this review, the general disease cycle of powdery mildew on apple are demonstrated and some basic features of powdery mildew epidemiology (such response of the pathogen to temperature, relative humidity, and rain as well as spore production, spore dispersal, diurnal patterns and temporal dynamics of the pathogen/disease) are also given on apple host.