Tomato Ringspot Virus & Tobacco Ringspot Virus– Both viruses can cause irregular berry development: poor fruit set and stunted shoots on infected grapevines. These viruses were vectored by the dagger nematode vector, and infected vines may result in death within 3 to 4 years. Resistant rootstocks, such as Kingfisher, Matador, and Minotaur, are available and recommended in eastern North America where ToRSV infects many fruit crops and broad leaf weeds species.(Tomato Ringspot virus, also known as yellow vein virus.)
Nitrogen deficiency – Deficiency symptoms appear as uniform lightening of green tissue, or yellowing of leaves when compared to vines that have received adequate nitrogen. Severe nitrogen deficiencies may result in small fruit size and poor fruit set. In the commercial setting, you may see the symptom development near veraison due to movement of nitrogen from leaves to berries. Nutrient test of plant tissue and soil followed by application of necessary nutrients is recommended.
Adverse weather during flowering –Poor fruit set may be a result of adverse weather during flowering. Extensive period of rain or cloud cover, strong wind, frost, and drought can all adversely affect cluster development. Check local weather history for possible damaging weather events.