Summing data from time points, sequentially in r

Question

I have data on yields of multipick crops across several weeks of picking. I would like to add the yields for each week to show an accumulation across the season.

Here is a small reproducible example of my data.

> mydata <- structure(list(cultivar = c(1, 1, 1, 2, 2, 2), harvestweek = c(0, 1, 2, 0, 1, 2), measure = c(10, 12, 8, 7, 0, 0)), .Names = c("cultivar", "harvestweek", "measure"), row.names = c(NA, -6L), class = "data.frame")

> mydata

cultivar   harvestweek  measure
1          1            10
1          2            12
1          3            8
2          1            7
2          2            0
2          3            8

I would like to generate a new variable that is the "accumulated yields". That is, I would like

cultivar   harvestweek  measure  accum
1          1            10       10
1          2            12       22  
1          3            8        30
2          1            7        7
2          2            0        7 
2          3            8        15

Is there an easy way to do this programmatically? Perhaps with dplyr?

I've tried cannibalizing and futzing with dplyr to calculate "changes from the baseline", using harvestweek 1 as the baseline, but I can't figure out how to get the additional observations to accumulate onto each other, sequentially, starting with week 1.