1. Why Muscat Grapes Are Grown Under Roofs Because They Dislike Rain
Muscat of Alexandria belongs to the group called "European grapes." These varieties grew well in the climate of their original habitats, which had little rain. In wet environments, they are prone to diseases and a problem called "cracking," where the fruit splits open [1]. Because Japan has a lot of rain, it is said that these grapes were hardly cultivated there before the Meiji era [1].
To solve this, farmers place the vines inside greenhouses. A page for business owners in Okayama City explains that because this grape hates moisture the most, it is always grown in a greenhouse [3]. Materials from Kurashiki City state that greenhouse cultivation of Muscat in Okayama Prefecture is said to have started in 1886 (Meiji 19) [2]. Is a roof enough to grow the grapes? Inside the greenhouse, night temperature becomes a major issue.
2. Why Farmers Heat Greenhouses at Night to Ship Grapes Earlier
According to materials from the Okayama Prefectural Agricultural Experiment Station, it is common to keep the night temperature at 18 to 20°C (about 64 to 68°F) or higher for Muscat of Alexandria. It is known from experience that if the temperature drops below 18°C (64°F), growth may slow down, and it may become harder for the fruit to set [4].
Why do farmers heat the greenhouses? The same materials explain that this variety requires a lot of care for its branches and clusters. Because heating allows them to separate work periods and aim for higher prices by shipping earlier, "heated cultivation" is widely used [4]. Kurashiki City materials state that the Funao district is said to be the top producer of heated Muscat in Japan, making about 40% of the national total at the time of the data. In this district, they succeeded in December heating in 1960, allowing shipments to start in May. Heated grapes are shipped from mid-May to late August, while non-heated ones are shipped from early to late September [2].
3. Why Only About 30 Grapes Are Left on Each Cluster
If a grape cluster is left alone, the grapes grow packed tightly together. A guide from Niigata Prefecture explains that leaving too many grapes causes cracking and rotting as they grow larger. Therefore, for Shine Muscat, the cluster is finished with about 30 grapes. This work is called "thinning" [5]. The number 30 is from the Niigata guide, but it varies by region and variety.
Before thinning the grapes, there is also work to shape the flower cluster, called "cluster cutting." Because hormone treatment must be completed within 3 days of full bloom, the cluster shape is prepared in advance [5]. Let’s count the work. In an example from the guide, a 10-are field (about 0.1 hectares or 0.25 acres) with 200 meters (about 656 feet) of main branches and clusters spaced 10 cm (about 4 inches) apart has about 2,000 clusters [5]. If each cluster has 30 grapes, that is about 60,000 grapes. Counting each cluster and each grape means tens of thousands of manual tasks for the whole field.
4. How Seedless Grapes Are Made with Hormone Liquid
How do farmers remove seeds? A children’s explanation from the Ministry of Agriculture, Forestry and Fisheries states that to remove seeds, a type of hormone called "gibberellin," which regulates plant growth, is applied to the clusters when the flowers are blooming [6].
The Niigata guide explains that for Shine Muscat, this is done in two steps. The first treatment is done from full bloom until 3 days later. This step removes the seeds. The guide notes that if this is delayed, the chance of seeds remaining increases. The second treatment is done 10 to 15 days after full bloom to make the grapes larger. Both times, the clusters are dipped in a liquid with a concentration of 25 ppm. The timing and concentration differ by variety [5]. The procedures are strictly defined because if the timing is off, the results change.
5. How Shine Muscat Was Created: 18 Years from Crossbreeding to Registration
People tried to create easier-to-grow varieties. According to a report by the National Agriculture and Food Research Organization (NARO), Shine Muscat was created in 1988 by crossbreeding "Akitsu 21" and "Hakunan." Akitsu 21 is a hybrid of the American Steuben and the European Muscat of Alexandria [1]. A variety that was grown under roofs was used as one of the parents.
The breeding goals were to combine the strength of American grapes with the quality of European grapes. Other goals included making them seedless with gibberellin treatment, having a Muscat aroma, preventing cracking, having good storage life and easy-to-peel skin, and keeping grapes on the cluster. They also wanted to reduce the labor needed for cluster shaping and thinning [1]. These goals match the weaknesses seen in earlier sections.
In 1997, the first selection was made. In 2003, it was named Shine Muscat and announced. In March 2006, it was registered as a variety [1]. The Ministry of Agriculture, Forestry and Fisheries also explains that it took 18 years from crossbreeding to registration [7]. This means 9 years from crossbreeding to the first selection, 6 years from selection to naming, and 3 years from naming to registration.
6. How Farmers Protect Grapes from Rain Without Greenhouses
Even without a full greenhouse, there are ways to protect clusters from rain. In Niigata Prefecture, it was thought that simple rain covers, putting caps on clusters ("kasa-kake"), and putting bags on them ("fukuro-kake") helped suppress diseases [5]. Materials from other prefectures also state that rain covers help prevent multiple diseases, while caps and bags help prevent late rot [9].
Okayama Prefecture produces more than 90% of Japan’s Muscat. However, according to a 2022 news report, shipment volumes have been decreasing by about 10% each year, partly partly because of the popularity of seedless varieties like Shine Muscat. That year, the expected shipment was 103 tons [8]. This is a figure from 2022 and may not be the latest.
7. Try It Yourself: Draw 30 Grapes on Paper and Check the Numbers
Try drawing a cluster shape on paper and draw 30 circles inside it without them touching. In real fields, if the grapes touch each other, it causes cracking and rotting [5]. Change the size of the circles or how they are arranged to see if the number that fits changes.
You can also redo the calculation from Section 3. If you multiply 2,000 clusters by 30 grapes, what is the total? For more details on the breeding history, the NARO Fruit Tree Research Institute report describes the flow from 1988 to 2006 and lists the full breeding goals [1].