There were complaints about poor fruit set of zucchinis this year. This was caused mostly by weather.
Most zucchinis, except for several pathenocarpic cultivars, have male and female flowers positioned in different part of a vine and need pollination/fertilization to fruit. Fruit development is a result from auxins released from developing seeds, which are developed from fertilization. Pollination is conducted by honey bees and other native pollinators in nature. When outside temperatures are low, pollinators are not active for pollinating. Also, under low temperature (or extreme high temperature) and wet conditions, male flowers may not release pollens or pollen tubes fail to develop. All these will result in failed fertilization that is necessary for producing seeds, which ultimately leads to fruit abortion.
Fruit abortion can be corrected by artificial pollination - choosing same day bloomed male flowers and female flowers, pollenize female flowers by gently rubbing anthers to stigma. Chinese farmers spray 10-25ppm (1ppm=1mg in one liter of water) 2,4-D to stigmas around 9am and have had very good results. When spray, make sure not to spray the fruit part to avoid fruit malformation. Please note that 2,4-D acts as a growth regulator, not an herbicides at this concentration. As a herbicides, 2,4-D cannot be used on zucchini.
Most of times, there are confusions between aborted rotten fruit and Choanephora wet-rot fruit. Caused by the fungus Choanephora cucurbitarum, Choanephora wet-rot is developed on wilted blossoms and spreads to attached fruit. Once infected, fruit rots rapidly and fungus mold appears on the rotted area. The fungus growth resembles small blackheaded pins stuck into the fruit. This disease is spread by insects, wind, and splashing water. Disease development is promoted by high moisture conditions. Normally Choanephora wet-rot does not affect mature healthy fruit but it is difficult to distinguish between fruit abortion and wet rot. There are no effective control practices available for wet rot. Fungicide sprays are impractical since new blossoms open daily and need to be protected soon after opening. Drip irrigation under black plastic will help.
REVISED: December 3, 2015