While rainfall has declined and become concentrated, temperatures have risen - ideal conditions for pests like looper caterpillar and tea mosquito to infest the light green tea shoots just before they are ready to be plucked for processing.
Use of pesticides and fertilisers has nearly doubled as a result in Assam's 800 big tea plantations, known as gardens, and the rising costs are making Indian tea less competitive.
As a result, firms in Assam are resisting calls from activists and student leaders to lift the daily wage of tea workers from about $2 agreed to recently, blaming weak prices and the doubling of crop expenses over the past 10 years.
Read MoreBig companies getting serious about environmental risks
Assam Chief Minister Tarun Gogoi, whose Congress party was routed by Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) in the 2014 general election, has sided with the workers ahead of state polls due early next year.
Courting the tea vote
State elections have national significance in India - Modi needs to win most of the state assembly contests in the next four years if he is to take control of the upper house of the federal parliament and ease the passage of his reform agenda.
Tea tribe votes can swing results in about a quarter of the seats in Assam, the country's main growing area, and the BJP has been making inroads.
In an interview to Reuters, Gogoi denied an opportunistic motive behind his call for the wage to be raised to about $3 a day.
"I had warned the tea planters about climate change but they did not take care for a long time," Gogoi said. "They thought it would be easy money. I can't allow injustice for tea laborers."
Assam Tea Planters Association (ATPA) Chairman Raj Barooah said they would examine Gogoi's demand but "there has to be a fair wage that can sustain the industry".
The average temperature in Assam has risen by 1.4 degrees Celsius in the past century and rainfall is down by 200 mm (8 ins) a year, said R.M. Bhagat, chief scientist at the Tea Research Association in Assam's tea hub of Jorhat.
"In the last 30 years we have seen that the magnitude of the effect of climate change is pretty high," he said. "Rainfall has gone topsy-turvy. There is either too much or too little water, forcing planters to use sprinklers on what is a rain-fed crop."
Several tea garden laborers and planters Reuters spoke with said tea factories in Assam now only run for about six months compared with round-the-year operations earlier.
Read MoreCalifornia farmers under attack for heavy water use
Less rainfall resulted in an 8 percent fall in tea exports last year, according to the Indian Tea Association (ITA).
India is the world's No.2 tea producer but is less export-oriented than other producers thanks to its big home market, and Sri Lanka has been extending its lead as the world's third largest exporter behind China and Kenya.
Learning to adapt
Labor accounts for 60 percent of the total costs for tea firms in Assam, whose prices last year were higher than those auctioned in Mombasa in Kenya, Chittagong in Bangladesh, Limbe in Malawi and Indonesian capital Jakarta.
Profit margins at Kolkata-based McLeod Russel, the world's largest tea producer, are estimated to have fallen to their lowest in six years in the year ended March 31, according to Thomson Reuters data.
To cut labor costs, tea companies like Aideobarie Tea Estates, owned by ATPA's Barooah, are exploring greater use of machines to harvest and spray nutrients or pesticides.
Barooah, whose company employs 48-year-old leaf plucker Ghatowar, her husband and now her eldest son, is also thinking of expanding into high-margin white tea made from tea buds.
Other tea gardens have moved to cultivating black pepper, turmeric, ginger, vegetables and fruit alongside tea, while Indian scientists are testing tea varieties that can adapt and survive in hotter and drier conditions.
But in the face of long-term climate change, that may not be enough.
"With rain so scarce, a day may come when Assam will not grow tea any more," said tea scientist Subhash Chandra Barua. "Planting a crop is fine but economic cultivation may not be feasible".