|dc.description.abstract||Asparagus ilarvirus 2 (AV2) occurs in asparagus crops in New Zealand and worldwide, but its effect on and spread within crops has not been reported in the literature. The causes of declining yields in asparagus crops, reported in recent years, are also not well understood. This investigation examined the incidence of AV2 within New Zealand, its effects on yield and its spread within asparagus crops.
By using an indirect ELISA method, that was developed for detection of the virus, AV2 was shown to be present in 93% of the 67 New Zealand asparagus fields surveyed and in 44% of sampled plants. Incidence was higher in older plantings, indicating that secondary spread of the virus had occurred.
A field trial with clonal plants showed that, for the years 1992-1996, AV2 infection caused annual reductions in mean marketable spear yields of 14%, 28%, 20%, 48% and 57% and increases in mean reject spear yields of 93%, 105%, 207%, 352% and 167%, respectively. Spear yields from AV2-free plants increased annually over the life of the trial and although yields from AV2-infected plants increased to the third year of harvest, they decreased thereafter. In harvest years four and five, AV2- infected plants produced fewer, smaller spears and fern stalks than AV2-free plants.
Transmission of AV2 by cutting knives, was investigated in a separate field trial in which the AV2-infected plants were placed in the middle of rows with AV2-free plants on either side. During 1992-1995, spears were harvested by cutting each row in one direction only. One block of 12 rows was cut in the same direction as the prevailing wind and the other block in the opposite direction. Plants were tested for AV2 annually. By 1995, AV2 had spread into 96% of plants cut after the AV2 source plants, indicating transmission by cutting, and into 26% of plants cut prior to the source plants, predominantly into female plants and those downwind of the source plants, indicating wind-assisted transmission.
The parental source of AV2 in infected seed was investigated by hand crosspollination between AV2-infected and AV2-free parent plants of each gender, growing in an insect-proof greenhouse. ELISA tests of shoots from the resulting seedlings showed that when only one parent was infected, AV2 was transmitted into 17% and 47% of seedlings, from male and female parents respectively, and into 95% of seedlings when both parents were infected. Infection of parent plants resulted in reductions in successful pollination, size and number of seed, germination and seedling viability. Of the adult plants originally free from AV2, which had been pollinated with AV2-infected pollen, 83% tested positive for AV2 the following year, indicating transmission of AV2 during pollination.
Because the same symptoms shown by AV2-infected plants have been reported as being characteristic of asparagus decline syndrome, in which asparagus crops become progressively less productive and are often uneconomic by the tenth harvest, AV2 must be considered to be a contributing factor to asparagus decline.||en